Jenny,
I have a tropical fish tank, one thing I noticed is that no matter what books and shops say the gourami in our tank does bully the other fish. I can understand the reason for putting the fish in the axie tank, but best not leave them in there for long as Leah said they do look very tasty those gills.
As a general rule, I would personally say that a good size is just under a foot per axie. so in a 2ft I would say 3 as a max, in a 4ft I would say 6 as a max. But in these conditions the water quaility should be kept a close eye on, as they may and are more than likely at some point to over load the bio filter.
As Jenny already has 7 in this tank I would just say PLEASE check the water quality every week if not every other week. As you could end up with some sick axies.