1 - Canti to Canti there will be no compatibility issues

2 - no, you can swith to V-type (linear pull) brakes or hydraulic (Maguras) they all use the same mounting. Canti levers are not compatiable with V-type (there are 1 or 2 exceptions to this, but you have to look hard)

3 - V-type offer better stopping than canti out of the box, although a well set up canti is just as good. V-type are a lot easier to set up well for the average user.