As someone who has messed around with live bearers for years, I've never had a problem with Mollies being finicky, and certainly never needed 50 gallons to keep more than 2 or had to do water changes every week.
I'm not sure where that is coming from, I've never heard that before, and it certainly doesn't match my experience.
The two things to watch out for with mollies:
1. They don't like high nitrates. A few live plants in the tank help keep Nitrates in check.
2. They like hard water with a high PH.
As long as you are doing those two things, they should be fine. Some species require a little salt in the water (Black mollies tend to have skin issues without salt), some are fine without it.
Note: This is in regard to your "normal" mollies and balloon mollies. I have never kept Sailfin Mollies, which are much larger than "normal" mollies.
I've always had good luck with mollies and platies. Guppies on the other hand have given me issues.