I would think once the fish reaches 3" or so you better have another tank set up or being set up for him. I think Midas/RDs grow quick. Get a fry from a good set of parents. At least that way you could keep him in a 20 for a bit and not feel like you gotta rush. What do you have going on?
idk. i have a 55 that has: 1 - texas, 1 - green terror both about 1.5" there is also 1- albino channel catfish, and 4 - silver dollars which the silver dollars wil be goin in the 90 gallon when they get bigger. i want to add another cichlid. i know they get big but my lfs is always happy to take big cichlids i bring in because they know they have been taken good care of. and i get tired of fish after awile.
So set the 20g up but put nothing in it and save it for emergencies. Get a Midas/RD thats close to the same size as the other fish you have. Put him in with them. If someone gets beat, grab the wounded and reheal him in the 20. Then return it to the pet store.
k. the 20 has been setup for a year. i can easily find a midas or red devil that size. which one is less aggressive midas or red devil. i mite pickup one of them and a jag tommorrow.