When I just had the five in my twenty? It still took them about 6-8 months to hit the one inch mark.
78-79F is a tad on the higher side for platies. They like it 76-77 or so.
A twenty gallon long with about a dozen should be fine temporarily, a dozen in there is going to be tight once they get around the half inch to the 3/4 inch mark. Once mine get too big to be eaten by adult sized fish, I put them in my 55 gallon tank. If you can, do like I did and get a bunch of smaller ten and twenty gallon tanks to split the groups up in.
Feed them at least twice a day, preferably thee times, suck out all uneaten food and poop you see daily with a turkey baster or something and add in treated, clean water. Usually just enough to give back what you took, which shouldn't be more than 2-5% of the water. That can help them grow too.
Also, it matters what you feed them. Baby brine shrimp, glassworms minced to pieces(otherwise they're way too big). tubefix worms may be a better choice since they're thread-like, though you can cut them in half still or just go with baby foods. A lot of green food like Romaine lettuce, maybe a bit of kale, the occasional pea or green bean...Emerald Entree is alright, they can pick at the greens for a while, hikari first bites is good, bits off an algae wafer(small chunks they can chew on once they get soft enough, suck it out after an hour or so, less if you see them eating too much), you can even hammer some algae wafers into absolute dust for them, I do this, and soak it before dropping some in for them. Wide selection is good, and though omnivorous, they like more plant-matter foods than meat-based.