It would depend greatly on your bichirs and gars growth rate. I'm not talking in general but per your specimens. I would honestly get senegals that are a bit larger than the gars especially if your going to put it in with a longnose. I would say you could keep them together for about a year, then I would be worried about the bichir becoming a meal in the not so distant future. You would be a bit safer with a florida gar. But the general rule applies, if it looks like the gar can fit it in it's mouth then most likely it will try a few times. For example my longnose is less than a year old and can easily eat strips of tilapia that are 7" long and half the gar's girth.