I used to have an Oscar, a big Red Belly, and a monster Pike Cichlid together and together they ate a 15" pond catfish overnight 2 days after we put him in the tank.
tough one, midas are tough SOBs no doubt about it, but the rhom has one NASTY bite! IF they do stick it out to be adults they will most likely be some beat looking fish once they really try to claim territory. Arent your midas from Jeff? i wouldnt risk it on a fish from him, too much quality and $ in his fish.
NO!! dont do it. a rhom will terrorize and kill your fish. rhoms are said to be the most agressive piranha of them all. you cant even shoal rhoms. they will end up killing each other. the only way you might even have a slim chance of them getting along is if the midas is 10+'' and the rhom is about 7-8'' but even at that i think one of your two fish if you chose to put it in the same tank will die. rhoms only get extemly nasty around 10''.