They are not known for fast growth. Perhaps from 3" to 6" in 1-2 years.
If you never see them, that's not normal. They have to come out, rarely, maybe very rarely but they should. If they don't, they are likely stressed - water parameters, lighting, cover, bullying, illness, etc. all play a role.
They are not fussy at all. Once settled in a tank, they will take almost any foods. If they never come out during the day time and/or feeding times, then turn the light off and leave the food for the night. If not eaten, remove in the morning but they should take to it much rather sooner than later.