Paul,
The main server here is a RHEL5 box, but I also run a laptop with FC12. I have had various FC versions working ok from FC4 - FC6 - FC9 - FC12 ( we only had FC originally when the the only restriction was the gcc and glibc versions)
The DB I use has been primarily PostgreSQL 8.4 on FC, although a copy of Informix 9.4 was runnable with some tweaks - all against Genero 2.11.x at the moment. The issues that I get (or have had so far) have been the same under RHEL as FC - but that could obviously just be luck and I haven't hit an area that causes a difference! So:
1. PostgreSQL 8.4 (Informix 9.4??)
2. FC12
3. Genero 2.11.x