I always want the owner of the new tank to "fall in love" with one fish, or one biotope idea, to set the theme. I have never set up a tank without having very definite ideas about what I wanted in it, so I am somewhat mystified by those who solicit suggestions of others. Anyway, you mentioned the sorority tank idea so here goes . . .
What I like about the idea of a sorority tank, is that this volume is the appropriate size. Lots of people think you can stick a whole bunch of female bettas in a ten or fifteen and have anything else but bloodshed! The only thing that concerns me is that the tank is a 45 tall. I wish it were a breeder 40 or 40-long. . . is this too tall a tank for the girls? I don't know. They may be okay, unencumbered as they are by the ridiculously long finnage of the males, or you could make sure that some floating plants and upward reaching driftwood would be available to act as resting platforms so the ladies don't have to climb so many stairs. He he he.
I say go for it, then look for fish that will cope with the same water chemistry and temperature. I am thinking of dwarf chain loaches, lochatas, striated loaches or even clowns which, granted, get large, but how many decades will that take?
Something in me doesn't trust the mostacembelus eels with slow-moving fish like bettas. Am I wrong? I think they are fascinating, but they get to be rather large creatures, and they definitely use that stealthy demeanor and big mouth for something!