[Q]

Dumbluck just modify in the race AI settings text file the line
Get Angry Over Allied Colonizable Planets := False

Then they should not get angry about all your nice planets. One race that already has this line set to "false" is the Phong. All races that have "true" will get angry about your planets, if they can colonize them. So it's no danger to make a treaty with a race that can colonize only Gas Giants, if you colonize only Rock planets. But beware of the moment when the other race develops Rock colonization technique!