How Many Platy In My Tank

i have 90L tank 24" x 15" x 15" i don't want to overstock, but of course want my full quota lol. The stock adviser here is a bit generous so i have visited another and have settled on a figure in between, and i would like (i think) your opinion on it. Just assuming i was having nothing but Platy's ( though i will probably be having tetras of some sort as well) according to my in tank filter, volume and area, i am told i can have 77cm or 30" of fish, so that's 15 Platy  (at 5cm) to my reckoning, (or 8 platy and 8 black neon tetras at 4cm) so go on folks, rain on my parade 

Oooh that's not so bad, thanks Col  Would it be possible to put two male Mollies in with my Platy's, or is there a temperature or salt issue, i'm sure my Lfs will say "they are all acclimatised" It would be nice to have two larger fish in the tank if possible, (less of others of course).