That's fine. =] I don't see a problem with doing a pair of GBRs.
A couple of things to consider:
1) temperature. GBR's prefer higher temps, 78 or above, probably. 80 is ideal. This might not be the BEST for other fish, but most can acclimate to it.
2) hiding/breeding places. They need their own territory and can get aggressive when breeding, so make sure there's plenty of room and you're not overstocked.
3) Proper acclimation. I dont' think fish NEED a specific pH, but you need to make sure they're acclimated to yours.
4) Water parameters. If you add them right after your fishless cycle, add them alone, so that your bacterial filter has a chance to catch up before you go adding a ton more fish. =] Also it'll give them a chance to be established and comfortable before adding tank mates.
Good luck!