I would like to report the following two issues:

First of all, not sure whether this is a bug, but I started a game with random opponents, and one of the AI opponents was the Custom Race!!

Second, this Custom Race opponent had aligned itself with Neutral. Then it wanted Subspace Rebounder from me, in exchange for some other tech. Needless to say, Subspace Rebounder is only available to Good races, so they couldn't actually use it. However, they still asked for it in the diplomacy screen, but it was not in the list of requested things on the left (my) side. There was still the other tech they offered in exchange. The result was, I got their tech, they got squat.

To make things interesting, they did the same thing again a while later, only with a different tech!