I'd just skip the RTM's if I were you. More trouble than they're worth. If you got a pair out of all this, then what? If you don't have enough room to keep all 6 right now, and in your best case scenario, you end up with a pair breeding in a 75.......then...... What you gonna do with a couple HUNDRED RTM's? In a 75. Fish that are notorious for not living well with each other, or other fish?
I saw some photo's of some a couple years ago, they do look cool and all. But, I just can't see dedicating a tank to a pair of RTM's. They're not THAT cool.
Now, if you're looking to buy a couple new tanks real soon, that's different.