Matching main colors gives more experience, so given a red and a green carbuncle at equal level, Horus will get more exp from the red one.
While Horus is pretty important, you're going to want more HP as time goes on and he's not going to be able to supply all that himself. Your starter dragon is pretty easy to evolve and can supply a little bit of that, as can the other subs.
Questions 3 and 4 can be answered by the same comment: if you don't need that evo material for something, sell it. The exceptions in my opinion would be dubmythlits, keepers of rainbow/gold, <color> dragon fruit, angelits/devilits, or jewels. If you're low on coins, regular monsters can be sold too, though I'd prefer feeding them to each other for more yield when fed to something you're actually using.
And combos just come through practice. For Horus in particular you'll want to eventually be able to do four combos in a turn, because he needs four colors to activate his leader skill. If your screen is small, maybe consider a stylus?
