That sounds like a good idea. It isn't realistic for the AI to stick with an alliance always, even if you're obviously taking advantage of them or influencing them to your side.
Alliances would be much more exciting if there was a chance the AI could break them if they were unhappy with you.

Well recently while playing on painful, the galaxy went through several MAJOR wars. The first of theses wars ended up with the complete destruction of the Terrans, Korx, and Thalans, that was only in the first month of the war. This cycle continued on for a while until the only civs that where alive where the ones that worked together(not allied but didnt kill each other) I was allies with the Drengin and the Torians, but they didnt really like each other that much. Also the Arceans had been pretty much by themselves over in the corner and had created a massive military. The yor where the only enemy we really had in common but they had really good relations with the Drengin. This lasted for about 20 turns then the Arceans attacked the Torians. I knew that the Drengin would help the Arceans, and that the Yor would come in somewhere and take out someone. the only way to end this was for me to ally everyone. This took vast amounts of money. I said all that to say this. I had to survive through alot of very violent wars to get to the piont where an alliance victory was even possible. If that isnt a challenge I dont know what is.

this may be a 'noob' question.. but can we make alliances with major races?

You can make alliances with both its just that the major ones are the only ones which count for the victory condition.

No, they're there for hugely lucrative trade routes where you end up with the money from both ends ;o)

You can do standard trade with tech trading off. Tech trading only cuts off the trading of techs, it doesn't stop freighters.

I personally use Minors as a planet-management system. I don't want to have to personally manage dozens of planets when I start invading, so I ally to a minor and gift them whatever planets I want to be safe from enemies.

To ally you need:

- The Alliance Tech (No, your potential ally DOESN'T need it)
- A certain level of relations (This used to be Close, but you can sometimes get it at lower relations now)

Then the Alliance option appears in the same place you'd find the Peace Treaty or whatever.