Hi,
I think the Ability to forge an alliance is a bit too powerful. It is rather easy to get even a stronger player to ally with you by using trade. When you do this with even a second one, you could just wait till they have wiped out the rest of the galaxy and you win by the alliance victory condition.
Of course It wont work all the time, but pretty often.Even with this condition deactivated, you could slowly steal your partners planets by using influence bases.
I think playing like that can be fun, but it is just a bit too easy to get the alliance, and your partners are to lax. They should be able to cancel the aliance like you can.
Erik