i just installed a new battery for my pedal, but am still using the adapter for in band practise (even though the battery is in it) in case, since sometimes i'm unable to plug it in certain rooms.
im just curious should i take it out completely before i need it, does it use the battery too whilst it's in there?
I'm not 100% on this, but I believe it depends on how the pedal is wired up. On an older pedal, I would say it probably drains it regardless. On a newer one, however, you should probably be OK to leave it in.
Haven't you heard?

It's a battle of words
If when you plug in the AC to the pedal while it's running on battery and it re-boots, the battery doesn't drain.

Or even easier, if you just plug in the jack into the pedal and it turns off, then you'll know the battery wont drain with the AC.
Nope, the battery will drain if there is a lead plugged into the input. It will be a slower process, since the battery will not be used to power the unit when it it's receiving power from the AC adapter, but when that is unplugged/turned off, the unit will drain unless the input is removed.

It's in the manual of virtually all effects pedals.
Take out the battery if you're using an adapter.
To buy:

Line 6 MM4
Voodoo Lab power
Mesa Boogie Single rectifier
Boss TU-3 Done
Shure SM58 Done