So, do some mods work more efficiently than others? Like IC2 vs. BuildCraft vs. Redpower. Or am I just retarded and they're all made to intertwine and work together?

dunno i just mine, guess that makes me a dwarf. NIKOLITE POWA FOR GMAN

lol.

You're a minion.

Greg tech changes recipes when certain mods are available and makes you use things from those mods. For example Greg tech requires rail craft steel to make a mining drill. It's meant to make shit more difficult, it's a way to nerf shit so you don't have a million quarrys and full quantum armor my the end of the first week. It's gay as shit if you ask me.

No, I think thats awesome personally. The longer the game is the more enjoyable it can be IMO.

So, I've only recently just explored the 'technological' aspects of the mods. (I tended to just stick to the magic stuff before.)

Efficiency seems to depend on what, exactly, you are trying to do.
Since they all seem to be self sufficient, you can get a good power grid up with any one mod. (IC, BC, RP, and ThermalExpansion.)
GregTech DID make things harder, but from what I can tell it only affects IndustrialCraft.
It didn't bother BuildCraft or RedPower.

Due to the multi use of mods, many of the mod machines seem to be convertable or interchangable. (Such as pipes working across most mods.)
So it really comes down to what you want to do, and what is easiest to do it with.

I find RedPower to be pretty easy to start up and get running. Or maybe that's because GregTech fucked IC up. Or perhaps just because I haven't touched IC yet.
So maybe it's good to use that to get things built up a bit so you can have the resources available for one of the other mods.

Who knows?