Hi and Happy New Year!
I would have to say that congos would prefer the softer water if given the choice. They are pretty sturdy fish, but some things they just can't adjust to, and a pH of over 7.8 would be something they would struggle with.
As for the rift lake and mollies... it's not the environment so much that I would expect them to have a problem with, I think they could be acclimated over just fine if it's done slowly enough, but what other fish are you considering keeping with them?
The most common rift lake fishes are cichlids, and these can NOT be mixed with mollys. Mollys and small cories would simply become food.
I'm not sure I would try rift lake conditions with the corys tho, I doubt they would survive for too long that way. Corys would prefer softer water than what rift lake conditions allow for.
Tell hubby that there is no such thing as too many fish :o That simply doesn't exist
I'm at my bare minimum right now at 26 tanks. (I have recently taken down another 4) Once we get moved and settled, we'll have 100+ tanks! :D