If two player factions ally and their infocards support that then it's up to them what they do and how they do it in their alliance. All I want is at least one, detailed RP post/thread stating that they are allied, what the goals of the alliance are, what the terms are etc.
At least then they can point to some proof of their ingame dealings here on the forums.
This I think is a very good approach. In terms of player factions in an alliance, they should dictate what happens between their ships, to an extent. For example, RoS or 101st allied with someone could conceivably get an ally access to the Outcast Battleship (I think the OC Dread should be restricted to ONLY Outcasts, which is why I'm getting the money to change the Caedo to a BS and get rid of the Dread.) but the BLS is smaller and has less access to the large capital ships. Mostly because they are more piratical and fast-attack strikes rather than the brute force that the RoS/101st use more often. I don't even think the BLS do have a BS, though they might. *Looks at Zapp.*
I think though, that any Battleship-class purchase needs to be fully RPd, even if they are allies. And of course, an RP post/thread for the alliance itself.