1) Head colour. Red cock + black hen will always result in all red unless the cock is also split for black.
2) Even getting a red hen doesn't guarantee that red will choose red, so you can't have like with like in a colony breeding setup as you have. You need to accept you're mixing head colours or separate the types of birds.
3) Unless your nests are well protected, I reckon this summer is going to be waaay to hot to get things moving now. I'm waiting a few more weeks before putting boxes in, I prefer my fried eggs to be chook not finches.
4) Many breeders advocate spare cocks to get things moving, others say only even pairs. Your birds may well dictate this for you. This is the same logic used to keep cocks and hens separate, it's always been my experience that the boys are keener than the girls

5) Gouldians bred in 2011 should be ready to go in 2012, but it may take some hens longer to get "the hang of it". The black beak will tell you when an individual hen is in season.
6) 2 boxes per pair is important, but so is space, at least 500mm all round each box, ideally up to 1m.