I say min for id shark would be 12' 6' 4' and thats 2,150 Gallons..... These fish are very active and wont stop moveing...... These fish get 3'+ would like to be in a group of 3+ so even bigger would be better..... The eel and BGK will be fine in the 55 for awhile the fire eel likes to have a spot to hide and would go great when u get that ply tank..... u might need to upgrade to a wider one but i think its alright if u got 2 ac110s on it.....
I'm curious if any of the people citing 3.5' ID sharks have ever personally seen one/grown one out to that size? Or are people looking up record lengths and listing them? Personally I've seen some big old ID sharks, that were kept and grown in 8x3 tanks, and that didn't make it past 30" even though theoretically in that tank they could have.
If you're really planning an 8x4 and you really want an ID shark go for it, you'll be providing it a better home than 99.9% of its buddies in the tank at the LFS. Otherwise the BGK and eel would do for awhile in say a 75 (BGK could stay there for life). I'd shoot for a 180 for a fire eel for life.