Raaa I'm just so happy that you made a thread here again. <3 Also that is a pretty badass team, Haxorus, Kyurem, Gyarados, Charizard and Salamence (you mentioned Bagon so I assume you like its final stage in the line, haha) all get my vote. (b'')b
Everyone else has been very helpful in linking you to some awesome PC sections, so I'll just be here to support what they say and encourage you to check out each and every one of them. :3 There's probably a sticky atop each section that you have to read first, so don't forget that! It's gonna be a lot more fun if you don't mess up, because believe me, I learned that the hard way back in 2008. u_u Also why are you very sad with having a full Fourth Gen Pokédex? I think that's pretty awesome!
Plus regarding your FireRed / LeafGreen team, you can always check out the
In-Game Team Help sub-forum to get the best advice, because most of the really skilled users who will know what they're talking about regarding team building don't really go here, they just wait for people to post there. ;D But I'll still give you some quick advice with the little that I know: I think it would be a lot better if you stick with one STAB move for each, because I noticed you had two with Umbreon having Dark Pulse and Payback, and with Espeon having Psybeam and Psychic. Perhaps you can even put a coverage move there, to not have to rely on switching so much. Also if I remember correctly, aren't Dark Pulse and Payback moves from Fourth Gen? Maybe you can switch strategies if there aren't good Dark-type moves yet, or at least replace it with something like Faint Attack. But yeah, that is what I think!
Anyway, welcome to PC, Raaa! See you around. :D