Looks like it is time to jump in here. A general opinion of mine on diplomacy is that plenty of people make too many friends and NAPs around the place - usually the more enemies you have then the more opportunity for conflict and activity. Some groups, dating back to vanilla, do have some weird diplomacy which usually overlaps with the "too many friends for no real reason" category - primarily unlawfuls.
Some examples would be the Mollys who seem to get on with a hell of a lot of people (two houses, getting on well with all Liberty unlawfuls, all of Rheinland unlawfuls, most Kusari unlawfuls), and a lot of factions' reluctance to shoot the IMG (Gaians, Chrysanthemums, Xenos) and to a lesser extent ALG (who have a fair few unlawful friends - Mollys to me seems pretty pointless other than Tyler and Nik/Jeni getting on well).
Things to consider with diplomacy include but are not limited to previous interactions, shared (potential) friends and enemies, trading possibilities, tech sharing, shared ideologies, not having the resources to fight them.
Now onto the Colonials, whom I obviously am rather involved with and probably can give the biggest run down for interest. Of course, =CR= diplomacy is rather shaky and changeable, so that makes it either a very bad or a very good example to use.
"We", "us" etc. now refers to the Colonials, both historically when I wasn't involved and currently as CR|. "They" might be =CR= or another faction.
We were previously at war with Bretonia after their historical shunning of us, leading to reasonable relations with the Gaians (maybe Mollys too, although they disliked us at the start of CR|) and Kusari. We later made peace with them and ended up at war with Kusari (I think for unrelated reasons), and now we've sided with them against Gallia, along with setting aside the squabbles with BMM over Tau-23. Their neutrality with the Outcasts isn't particularly liked but we're alright with shooting GRN first in Leeds - Bretonia shouldn't take the Outcast relations any further than neutrality in my view.
The Gaians aren't really around to interact with, but I'd say they are definitely against us either as Gallic pirate lackeys or ideological environmentalists (the two sides of the movement) since we are one of the more terraform-y groups.
The Mollys were lobbied for neutrality to allow the Movers to safely trade in Bretonia, and since we didn't fight in Bretonia at the time things were ok. However, their increasing cooperation with Gallia and our increased presence in Bretonia (which has caused a couple of incidents) will likely lead to hostility which I am ok with. The economy has recovered enough to cope with some extra pirates on us, and some extra targets in Bretonia is always fun.
Rheinland is a decent enough neutrality, despite the small dispute over Omega-7 a few years ago and some of their hesitations from Gallia's opinion about it. However, I doubt that'd come to much unless they ban Bretonia too.
Kusari remains our enemy, although we don't particularly fight them inRP because of both our small size of fleet and ooRP because of not very much overlapping ZoI. After a fair bit of cooperation through this Rebel RP I hope to come to some sort of ceasefire since it is mostly just annoying KDS shooting my transports and trying to get screens to ruin the IMG's standing with both Kusari and Gallia - this would stop and is more of a pro than the con of losing only a small number of pews.
The SCRA, forming after the Colonials left for Crayter, don't suffer the same "you engaged without RP at the Balma Treaty Massacre" hatred as they do from Alliance houses. Neutrality is fine, and them losing our ships keeps it hidden since they krieg some of our friends.
Hellfire Legion is another neutral unlawful, although I don't know the history for this one. Common enemies in the Outcasts and their Liberty allies and a shared proximity to the border worlds work well enough. Kind of annoying that they are disbanding but inRP we'll still be getting a fair chunk of resources from Guadalajara mining after that is set up. ooRPly it means I can get friendlier with Liberty a bit easier!
The Order and Colonials had some old coop against the Outcasts before my time, and have had a few recent (both impromptu and prepared event) ones too. With them moving into Inverness with the Atum and Tau-117 being made into the more accessible Drake this will likely increase.
We've also been neutral with the Core since I'm not a fan of playing both sides. If they get pissy about the Order, I'm not all that bothered - we've had a look at their public databases (plus a little extra data thanks to Prudence) and can move on without very much mess.
The Council is a bit tricky, relations vary on each side and over time and it also being based on some ooRP reasons. I agree that Fallen Grace had some mistakes in it, mostly from Jeremy wanting to include CR| which required a bit of a stretch and left out the Council. There is also a bit of imbalance with too many people on the same side - while the Council have more ships and Gallia in their ZoI, they can't really use that last part so are mostly a Tau faction. They lack the history of the Colonials in the region and a strong leadership, so fall behind in activity since you can do most of and more of the same actions on CR ID.
I prefer to think of it as the Gallic Council who have a large defensive fleet in Champagne/Artois and a more subtle presence around the rest of Gallia. They are kind of pissed about Fallen Grace and don't like us being in there (which we don't have all that much reason for, other than sneaky CIS stuff or transports moving Refugees). On the other side of the minefield you've got the Sirius/Roussillon Council (note the two S, one I, two L spelling, devs!) with a similarly-sized active fleet to ours (with some of ours inoperable until 4.87) without the infrastructure of Sabah. Although they have the Gallic lot's annoyance, they still cooperate a lot against the Outcasts and in the TAZ/C/CR triumvirate in our region of space. We get on with them, and the Gallic lot are more just a useful distraction for our enemies like the Exiles (distrusting neutrality, which will likely continue).
The Corse friendship was also based originally on largely ooRP reasons - there weren't many other Gallic factions active when we were raiding the house. While I do admire the faction, they were and kind of still are a bit too close to the anti-Royalist side than I'd have thought of a quasi-lawful faction. That has left them to consider us allies while we just had them as friendly, and getting on well with the Council while they didn't fully distrust the Corse. That being said, C-UC cooperation can still happen; nothing too important though. You may be confusing them with the Gallic Junkers who are much more disliked by the Council.
Thankfully we've settled to them being an anti-Outcast assistance outside of Gallia and a source of intelligence and supplies through their connections to the Gallic lawfuls. You've exaggerated the problems here, CR gets on with both sides and the Council has no real problems with the Corse (lower end of neutral-friendly both ways) so there would be no problems with any cooperation as any pair or all three together.
So hopefully others do have a similar attitude to their diplomacy other than "make friends with everyone". Don't be afraid to make more enemies where you can cope with it (ooRPly, but inRPly to some extent) rather than having allies all over the map that don't do anything for you. Shoot the IMG once in a while: all you have to fear is our defence wing!
