i just came across some mollies myself..1 female who i pregnant...i dont know much about fry but i do have some of her offspring from a batch a few months ago.as i have had them for a few weeks now..1 of the youngest swims kinda like how u mentioned.and kinda like a submarine going up atop...he swims with his head up and in burst..then just kinda sinks back down with his head pointing up.i think thats just how he is and nothing to worry about..mines a few months old.and plays with the others..
some1 referred me to your post due to the fact i too have mollies and to help get a little discussion going..but from what u have described..its just how he is..and he should be fine.as long as he is eating..adding salt to their tank wont hurt..but dont add too much too fast...also a bigger tank might help...really its advised for something bigger than a 20 gallon.though i had 9 in a 10 gallon from the woman i got it from.1 died,the female almost died the other day.surprisingly he is still alive..and they are now in my 20 gallon hex with 4 rosy red minnows and all as happy as can be..
since u have about 2.5 gallons of actual water..add about 1/2 teaspoon for now and see how they do.that will also help protect against external parasites..
2 female VT donna and pirahna
1 male VT keiichi
4 rosy reds no name yet
2 mystery snails Gary and Patrick or pat if female.
1 filter sponge named spongebob
7 black mollies