Your goal is possible but definitely hard, expensive, and time consuming, + tons of breeding (unless you're lucky).
1. giant x giant doesn't = 100% giant (fact). So giant x regular = 100% regular size BUT 50% geno. Breed that, you get half giants (unless you get really lucky). Breed that again, you may get full giants. ..... question is; which pair do you breed? Which are the giant genos or the half giants with full giant genes? If you want to play it safe, you'd have to wait until they're big; 6 - 7 months and breed the fastest growing and the largest.
In other words, you need as many fry survive as possible. Choose the fast growing (which might not have giant genes) and breed them. You have to breed as many pairs as possible. If you're lucky, it can be done in 4 - 6 generations.
2. Giants need tons of food to get them grow rapidly and reach the 12cm BO mark. Imagine feeding hundreds, or even thousands or fry 6 - 9 times daily..... it's very expensive and time consuming. I'd say invest on an expensive HM giant pair. Experiment with their fry and work your way for colors ..... you will still have the true giants to breed back to when you need them.
3. Not sure about this one, but I haven't seen HM giants with fins as long as regular HM. Anyway, if you power grow bettas, their fins will not develop as good as if you grow them regularly. You can power grow PK and get equally gorgeous fins but HM,s tend to have shorter fins. So if you power grow HM giants, their fins should only grow half of their body length (max). I'm not sure if fins could grow later on, but my previous HM's never did.
I hope I got the words right, because it was hard to make it simple and short.