In all I have Bichirs in 4 tanks:
1. One 18" Endli (in ~200G)
2. One 10" Ornate (in ~30G) : "Under observation"
3. One 8" Delhezi (in ~55G)
4. Four 5" Endlis and One 4" Ornate (in ~30G)
I think the growth rates for the larger bichir are going to slow down soon allowing the smaller ones to catch-up.
I'd move the four smaller ones into the 55g to try get them growing a bit faster. I've noticed polys seem to have growth spurts when moved to bigger tanks. I grew out 4 dels from about 2", started in a 12g, then into a 30g, then into a 70g, then into a friends 1200 litre tank, and every time they were moved they had a spurt. Same for my ornate going from an 80g to a 160g to a 210g. Bigger tank + more food + more water changes = bigger bichirs. Not sure how big they'll need to be to go with an 18" endli thats used to living alone though...