20-30 in a 5 gallon could be a lil much. They are perfect for con's and could be a lil small when oscar is full grown. Best thing to do to make them adequate feeders is to feed them what you'd feed your cichlids. High quality flake, pellets, etc.
I was trying to do the same for my Grammodes, but due to his aggressiveness i had to move a con and Rainbow cichlid to my 29g, which i was gonna use to breed guppies.
I started with 2 males and 6 females almost a year ago. The first batch of fry appeared just about 3 weeks to a month after being in my tank. Had it good for about 2 months, feed fairly large juveniles to my fish. Then i had to move fish around. THey continued to breed even with 2 CA's(albeit smaller, both around 3.5-4") cichlids. They did however kill/eat both males.
I currently have 1 male and 5 females. My current male is a full grown fry from my very first batch. I have atleast one female form subsequent batchs. i still get fry every few weeks, atleast once a month. The Rainbow cichlid and Convict keep population in check. Too well actually lol they've been clearing out fry pretty frequently none have lasted past 2 months.
Unsure how long you'd grow them out before you give them to your cichlids. But the key if you're grooming them for feeders is food must be high quality and good nutritional value for your cichlids.
lol sorry for such a long post but also to add definitely will want plants fake or real, guppies are known for cannibalizing their young.
I gotta admit when i seen my former large male eat guppies fry, i was impressed.