Convicts will work out fine for the midas and terrors. They'll breed easily once you establish a pair.
I'd consider the rcs for several reasons.
- You'll only need one tank. With convicts you'd likely want to strip the fry and powerfeed them in a seperate barebottom tank to bring them up to size for feeding ala discus breeders. Otherwise you may run into slower growth and your pair might stop spawning.
- If you're successful starting a colony with 10-20 shrimp, you'd be deep into the 100+ range inside 2 months. Once you got a feel for the colony size, you'd be good to net them out at will so long as you didn't feed berried females and kept the tank shrimp only..
- They won't be overly substantial, but honestly convicts won't be either unless you crowd the living mess out of the 2nd 20 growing them out for a month or two. You'll also risk losing any fry you add in from the parents tank from their next spawn to keep them going. Since you're more or less looking for conditioning foods, anything that moves quickly and is nutritious works.
- Biggest winner in my mind is excess. If you decide you've got to many convicts you may have some issues even giving them away.

If you have a lfs that'll trade in they'll likely be interested in the rcs, or simply sell on Craigslist for $1-2 each to locals...
Drawbacks to the shrimp over convicts are their max size, preference for a planted tank (Though even low light stem plants will do just fine), and being more fragile in terms of water quality.
Currently feeding convicts, but I've just found a local offering rcs that'd happily live in my planted tank.
Hopefully the wall of text helps.
