yes, keeping the 50 amp sub panel is fine. not sure exactly what's in it. it sounded to me like 50 amp breakers going in and 50 amp going out. wiring the outlets directly into a 50 amp breaker is a bad idea.
my experience with outlets, a cheap outlet is a cheap outlet. doesn't matter so much if it's rated at 15 amps or 20. I've melted 20 amp cheap outlets with small heaters and air conditioners.
for my grow room, I'm getting a 400 watt lamp, and running a dedicated 20 amp 12 gage line to outlets, with 1 gfi outlet protecting everything, as well as an arc fault breaker.