Its easiest if there the only fish in the tank. No competition. In my experience most will eat frozen if given time. You need healthy fish to start tho. If there bellys are sunken or they look thin its generally too late. These are one of few fish I will buy rt when they arrive at a lfs. Most just don't keep them in ideal conditions to thrive.
Now to the fish themselves, male mandys have a high dorsal fin. Scooters a large flag like dorsal. Don't mix males of the same kind. I keep mine paired or one male w a few females. Do not mix spotted mandys with green mandys. Spotted are very aggressive w other mandarin. Fine w scooters tho