I didn't get to do the testing I wanted to last night, but I've noticed when you discover someone's sign, sometimes it says the two signs are compatible. So far my Taurus girl has met sims with 6 different signs and 3 of those are compatible. What I was going to do, was run a few tests with custom made sims to see if relationships improve quicker with compatible star signs. Haven't seen any incompatible signs yet.
Norma