The 30 gallon would be fine for now. You don't want to go too big with a juvenile gar. Just ensure you have larger accomodations as the gar grows. A tank that is two feet wide should be considered the minimum for an adult florida gar.
As far as tankmates you want to avoid anything that will pick on or outcompete the gar as well as silvery fish. Most cichlids, many characins, silver arowanas and catfish are not advised for this reason. Bichirs, african arowanas, sa lungfish, other gars are some of the tankmates that tend to be recomended. I have had good experience with doradid catfish. Just be sure the research the individual fishes needs.
Here is a thread that goes over some members tankmate experience,
http://www.monsterfishkeepers.com/forums/showthread.php?t=392233