I'm definitely no expert, but I think having strings too tight can make them just as floppy as having them not tight enough. I restrung my Dollmore girl to shift around some knots, but doing so made the string in her legs a lot tighter and now she's extra kicky. She can still stand pretty well on her own (as long as I don't touch her), but when I try to sit her, her legs get all twisty at the knees. You can try tightening the knots and see if it helps.
My Puck is a bit floppy, too. He tends to list to the side because of his upper torso joint. And he tends to flop over at the hip joints, too. My new (used) girl stands pretty steadily but I think her last owner had put some hot glue sueding in her joints to help steady her.
Really, I think it could be because there are SO many extra joints that minifee flop a bit more than other MSD. My Kid Delfs don't flop nor did Momo before I shifted her stringing. You get some great posing out of minifee that other MSD can't do, but you sacrifice extra stability for it. It's kind of a trade-off, I guess.