For ropes, I would definitely get 2 or 3 (I'd go with 3, some people say 5+), they are a lot like kuhli loaches in which they are happy with their friends.
For sens, I also would go with 2, though you could probably do one. I've noticed though since adding Namine, Nim has been certainly a lot more active. They constantly follow each other and zip about, and they hang out with each other a lot. As they get older, they like to have their own space, but even in videos where the bichirs have tons of territories, they tend to hang out near each other. I got my Sens both from Petsmart for $7, they were labeled as "dinosaur bichirs."
If you want both ropes and sens, you could probably do 2 ropes and 1 sen (or another bichir species), since they are both polys. I was reading more about ropes, and their growth rate is fairly slow, so if you get 2 8" ones, then they will stay around 14" or so probably for at least a couple years.
As for eels, a lot of peoples say their peacock eel hides a lot. Mine NEVER hides anymore, he is always out and about, nothing scares him.
Something to keep in mind is when choosing what to get is that bichirs typically are pretty hardy, (the sens and delhezis are typically captive bred) and easily found (sens at petsmart, delhezis are pretty common at LFS for anywhere between $10-$60, etc.) and easy to feed, they aren't terribly picky (live black worms, frozen mysis shrimp, bloodworms, ghost shrimp, small earthworms, frozen beefheart, etc. and hikari carnivore when younger, hikari massivore supplemented with bits of tilapia, frozen shrimp, large earthworms, other fish fillets, crayfish, etc. as they get larger; mine right now are on a diet of blood worms, mysis shrimp, brine shrimp and soon the hikari carnivore once it gets here). Some people say they can jump, but mine didn't show any interest in jumping out. When younger, bichirs like plants to float in so it's easy for them to get to the surface to breathe air, and it's best to keep the water level a couple inches lower even as adults so they can breathe the air. Bichirs are always out and about though, they don't really hide. They will eat tankmates they can fit in their mouths, so you'd want to get babies so your tetras wouldn't be at risk (or at least a lower risk)
Ropefish are ALWAYS wild caught, so they are suspect to more diseases, esp internal parasites. They are harder to feed then bichirs, typically preferring live foods, will take frozen, but don't like the prepared stuff like the pellets really. After reading some more, 75 gallons seems to be okay for ropes, ~15" seems to be the average length
They are EXTREMELY good escapes artists, be sure to COVER ALL HOLES so your tank is completely covered or your will find a little snake on your floor. Some peoples ropes hide, but a lot of peoples ropes are super active once adjusted to their new home. Ropefish cost around $15-$40 depending on where you go. Ropefish will eat their smaller tankmates, but with your stock, you should be good.
The spiny eels (at least the 3 I listed) are wild caught, the peacock eel the most common (can be found at petsmart) and need either sand (I recommend pool filter sand, NO RINSING!) or very fine gravel so they can burrow. They can be hard to feed, prefering live food and eventually accepting frozen foods (for the zig zag and the , but not taking to pellets. They have the smallest mouths out of ropes and bichirs, so though they will eat smaller fish, they have to be pretty small, like neons or guppies. They tend to hide a lot, but you can get that oddball that is out all the time. They are really good escape artists though like ropes, so completely cover that tank.
EDIT: Bichirs will have the least trouble competing for food, but they still are slow eaters, rope fish are next in line for having trouble competing, eels will have the most trouble.
If you don't want to worry about your fish hiding, I'd go with 2 bichirs, or perhaps 2 ropes and 1 bichir.
Bichirs HAVE A TON OF PERSONALITY though. I'm sure if you work on it, they would eat out of your hand. Talk to any bichir keeper, we all love them. Once you go bichir, you never go back!
