my tank is small around 20...saving up for 150...
Hello Again
Size of birchirs depends on species.
Some stay small some grow big and some turn into monsters. LOL
Most you see in pet stores are the small species.
I had to go to a local importer to get a Ornate birchir.($40 for 4"

)
Ropefish get long not big...LOL
They also grow slow IMO.
I have had 2 and they live in peacefully together.
They will fight over food like worms,great fun to watch.
I am sure you can keep one or even two ropefish.
This would limit the amount of fish you could put in that tank.
Only a couple or 3 fish at most.
I keep 2 adult fish per 20 gal in my fishroom unless it is small fry.
(depend on the species of fish in the end,some stay solo

)
You could research while you save money for a bigger aquarium.
This is a fantastic family of fish.
Good Luck
Members of the genus Polypterus
There are approximately 45 species in this genus:
P. aethiopicus · P. annectens · P. ansorgei ansorgei · P. ansorgei delhezi · P. ansorgii (Bichir) · P. bichir (Nile Bichir) · P. bichir bichir (Bichir) · P. bichir katangae · P. bichir lapradei (Bichir) · P. bichir lapradii · P. bicker · P. birchir · P. bichir subsp. katangae · P. delhezi (Barred Bichir) · P. dolloi · P. endlicheri (Saddled Bichir) · P. endlicheri concicus · P. endlicheri congicus · P. endlicheri endlicheri (Saddled Bichir) · P. endlicherii congicus · P. endlicherii endlicherii · Erpetoichthys calabaricus · P. faraou · P. senegalus subsp. senegalus · P. endlicheri subsp. congicus · P. endlicheri subsp. congicus · P. retropinnis · P. mokelembembe · P. ornatipinnis (Ornate Bichir) · P. ornstipinnis · P. palma (Marbled Bichir) · P. palmas (Marbled Bichir) · P. palmas buettikoferi · P. palmas congicus · P. palmas palmas (Marbled Bichir) · P. palmas polli · P. retropinnis (West African Bichir) · P. retropinnis lowei · P. retropinnis retropinnis · P. endlicheri subsp. endlicheri · P. senegalus (Gray Bichir) · P. senegalus meridionalis · P. senegalus senegalus (Bichir) · P. teugelsi · P. weeksii (Mottled Bichir)