For 1) and 2), they depend on how often you are willing to do the water change. (and of course, I'm assuming you have enough filtering capacity).
My 10g breeder tank has 5 female platies, half of the tank is full of java moss. Absolutely no water parameter issues. I only do weekly 20% water change, and I try to pick up any fries that are about 2 weeks old into grow out tanks. I can always see more than 10 little fries in this tank, most of them are under 2 weeks old. The tank is driven by Aqua Clear Mini and a sponge filter. Oh and this tank also has 2 otos (In order to let java moss grow, I have some lights - that must be causing the brown algae formation).
My 15g grow out tank has about 30 growing fries at any given moment, varying in size from about an inch up to 1.5 inches (big enough to sell to LFS). This tank has Aqua Clear 200 and a sponge filter, along with some java moss and java fern. Again, never had any ammonia or nitrite spikes and I guess the plants keep nitrate level low. I do 30% water change about twice a week, with thorough gravel vac. Obviously, this tank is overstocked, and I don't keep any fries that are larger than 1.5 inches - so far, it appears to be manageable.
So depending on how much work you are willing to do, 10g can acomodate (assuming 1.25 inch average size fries), 10 fries (very comfortable), or over 25 with lots of work (frequent water changes).
