haven t read the whole thread - actually, i haven t read a single post that time. - but the answer to the original question from my side would be:
there is no answer in general. an alliance is not like another alliance. they have different reasons, different factions involved. for example, the order is allied to the corsairs - that is not cause the order felt lonely and wished to have a good friend to talk to when they felt alone in space, but they forged that alliance, cause they were in dire need of resources and combat training, while on the other hand, the corsairs needed the technical knowledge from the order.
then again, we have alliances like the Farmers Alliance and the gaians, who somewhat share a common cause. They hardly ever meet each other, and if they did, they d prolly realize that they re not at all too similar, - but they are allied, cause they feel that they are somewhat pulling on the same end of the rope.
For example - i think its perfectly OK when a Gaian wishes to use a FA weapon ( ships don t matter, cause they both use the same ships ), but it feels strange when a Gaian wished to buy a titan from the corsairs ( the other allies ).
so one cannot make a rule about what allies can do and what not, but has to look at the reasons why the factions are allied and how likely it is that they not only share assistance and knowledge, but also precious technology.