The ABF will have no problems living in a 50 gallon tank, but the clown loaches will slowly outgrow the tank and will need to be placed in something much larger
Bossibly you could keep 1 or 2 clown loaches in a 50 gallon. Butterfly fish only need 30 gallons so you should have no problems. You could probably put in 4 or 5 AFB's in a 50 gallon.
You can easily keep about 3-4 young clown loaches in there until they grow up (they grow very slowly, about 1 inch a year over a period of 6-10 years). And for the middle I would suggest a BGK, it's an oddball.