If by Ruby or Sapphire you mean Omega Ruby and Alpha Sapphire, yes. However, he will be restricted to only using stuff that existed already in X/Y, because Game Freak are a bunch of douchebags who don't want to update the older game to share the same stuff (don't worry, there's not a lot of new stuff at all). (Edit: oh and about 3v3 and matchmaking, yes, there is.)

I'm sorry for your loss, but as far as I know there was a gap between Game Boy and Game Boy Advance where you couldn't transfer Pokémon, so Blue and Silver are lost in time I'm afraid. I heard there could be a way with some GameCube "game" that stored your Pokémon or something like that, but I'm not sure it's true and even if it is, you would have to get lots of games and consoles to transfer over your Pokémon from generation to generation. Not worth it I guess.

Look, I'm not sure where exactly to look at for this stuff. There's a few places I know but I would just google it and try to find the best explanation... which is the one I'll be giving to you now.

IVs, short for Individual Values, are hidden numbers a Pokémon has for each of their stats (HP, Attack, Defense, Sp. Atk, Sp. Def and Speed). A Pokémon can have between 0 and 31 IVs in each stat, with each IV making their final stat increase by 1 at level 100. IVs are usually random every time you encounter a Pokémon, but in X/Y there is a place in the post-game called Friend Safari, where every Pokémon you meet is guaranteed to have two perfect IVs (and since X/Y every legendary has at least 3 as well).

EVs, short for Effort Values, are also numbers that used to be hidden (now they can be seen in the Super Training tab) for each of a Pokémon's stats. They range from 0 to 252, with every 4 EVs increasing the final stat by 1 at level 100. Furthermore, a Pokémon can only have up to 510 EVs total, which means if you max out two stats, there's only 6 EVs remaining for something else. EVs, unlike IVs, are not a random number permanently assigned to each Pokémon, but rather something they acquire over time and training, much like the Exp. Points. Everytime you defeat an enemy, your Pokémon will get a certain amount of EVs (how many and in which stat depends on which enemy you faced), and you can increase how many they get with the Power items and Pokérus.

Um, I hope I explained it well enough. If you're getting into this stuff, you should also know how the Natures work, but that's relatively simple in comparison. Either way, I'd recommend you to stay away from all of this at first - just play the game naturally and have fun. The competitive side is better enjoyed after you're done with the story.

Edit: holy crap this came out longer than I expected.