LMAO, another victim to MTS.
Word of advice, slow down a little bit. Between the four RD fry and the flowerhorns, you're committing to a lot of nasty, large fish in the future with no future home for them. It's kinda like puttin' the cart before the horse, ya know what I mean? It might go well and ya might get the larger tanks up before they need them, but then again circumstances could prevent or delay that as well.
A male RD will easlily reach 10" and possibly more (mine is 12"), ditto for the flowerhorns. The males can also be very abusive to mates when they aren't gettin' the lovin' they think they deserve.
A large enough tank to allow the female places to hide and several escape routes, with decor breaking up the line of sight, is a must for these fish in a pair set-up, or the male can potentially kill the female. The larger tank ya can give 'em the better.
Can't tell ya a thing 'bout flowerhorns, other than they are a man-made hybrid. I'm kinda prejudiced against abominations...I mean hybrids, so don't pay much attention to 'em myself.
If yer serious 'bout breedin' 'em, ya may not want a pair composed of fish of different strains. Flowerhorns have developed into some pretty specific strains with specific characteristics defining each strain, just like goldfish.
saint_felony has a flowerhorn or two, and has done a bit of research on 'em. He may know a bit 'bout sexing them. I know the males get large nuchal humps, but how early they start to develop, I don't know.
So many fish, so few tanks.
"In the fields of observation chance favors only the prepared mind." - Louis Pasteur