Just 2 quick questions about pick-ups that I can't find the answer to anywhere.

If you replace all your pick-ups, do you have to get different types for your treble, midi etc... or is it purely on the placement of the pick-up.

And what is the general view of single coil EMG's and would it be a good idea to use 3 of the same. I'm a complete beginner for guitar modding so I've just got a Squier Strat for my first project, but I've checked measurements and the pick-ups I'm looking at are going to fit.

Thanks for your time and sorry if there's a previous post that already answers this. Don't flame me :P
If, by your question, you mean do you need to get different pickups for each position, people generally use a lower output pickup in the neck and middle positions because the vibration is stronger in those areas. But if you're using EMG single coils, you probably don't have to worry. Are you using actives or passive?
Then there's this band called Slice The Cake...

Bunch of faggots putting random riffs together and calling it "progressive" deathcore.
Stupid name.
Probably picked "for teh lulz"

Mod in UG's Official Gain Whores
Passive EMGs? That's a waste - might as well buy some normal passive single coils, those passive EMGs are nothing special.

And it's fine to use the "same pickup" in all three positions. Some people like the bridge pickup a little hotter, but the difference in neck/bridge output can often be balanced by adjusting the height. The tone will differ mostly because of the different placement of the pickups, as you surmised. The strings ring more freely as you approach the 12th fret, and less freely at the bridge.
Just thought - will i need to stick new pots in cos of the higher output than the standard singles?