Your tank size is fine for those number and sizes of fish for now. But I wouldnt try the lobster thing. I have had many, and the only ones that can survive even with convicts and texas cichlids are wild ones or the large ones I buy from the baitshop (which are blue, by the way). I once saw my 3 inch Texas hit a 4 inch lobster hard enough to rip the side of his shell open...at which point, the bichirs went to town. I have lost 4 lobsters that way. THe only way the one in the cichlid/bichir tank lasts now is because she's a real b*@&h and nothing messes with her.
The only other thing that I will say about blue lobsters is that they (most) are only a mutation among the P. clarkii crayfish, and these are the ones that most baitstores sell. The ones I have now are almost 6 inches long, are blue, and cost me $0.39 a piece. Maddening, since I have had several of the $25 blue "lobsters" before that were much smaller and less robust...