1) You'll want the bore size to match whatever paint you use most, medium bore/medium sized paint. If you use different paint all the time get one of the barrel sets that has all/most bore sizes you'll need (freak kit, CP pro kit). Paint/Barrel chart --> http://www.paintballguru.com/paintmatch.htm
2) The barrels are made for the different markers ('cockers, angels, spyder) because they all have different threading.
3) Their are different kinds of bore sizes for spyders just like any other markers.