They are not picky but they will not compete with other more active fish for food.
They will starve often from fear...The are 'micro-geophagus' meaning small earth eater.
They like to chew and spit their food till it is small and easily swallowed.They forage around the bottom for food ,but will not survive on scraps or if all the other fish eat all the food before them.Mine we eagerly [like piranha ] accept flake and pellet food like NLS...They splash me if I don't throw them enough food quickly,but they are the only fish in their tanks.
I breed and sell rams so like it or not my advise FOR ALL who wants rams is don't if they are not going to be the #1 fish in the tank. It sounds like you added to an established tank.. Who is in the tank they just moved into?
They may grow in well or be buying time depending on who is already in there...