Some of the early Mongoose MTBs were really nice. I've got a chrome Hilltopper with everything stuck laying around here somewhere (dug it out of the weeds at a garage sale).
FWIW just one datapoint on sizing, I typically ride an 18 inch MTB and a 22 inch racing bike. I recently converted my 21 inch (center to top) Cimmaron and am pleased with the fit. I do have just about zero standover, similar to my road bikes, as I have really short legs, and a long torso.
I never would have attempted the conversion with a 21 inch MTB, but since I had it, why not..
You can see my 18 inch Univega on the first posting on this thread, lots of seat tube showing, while my Cimmaron appears later. I'll put the two pics side by side. In both cases, I used the stock stem.
18 inch Univega, with lots of seat post!
21 inch Cimmaron, with just a handful of post:
bill