1/Can the First Strike FSC use larger magazines?
Yes, the First Strike Compact will accept T8/T8.1 magazines which have a larger capacity and can use the 12g Co2 cartridge rather than the 8g.
2/What is the velocity of the First Strike Compact?
The FSC has an adjustable velocity, but is typically used around 285fps-300fps
3/Can I use the First Strike Compact as a main marker?
That depends what type of paintball you are playing, however the FSC is designed to be a sidearm more than a main marker.