After modifying few weapons for my character builds, few questions came to my mind

1.Can you give the gun more the one additional clip.
2.Extended Clip do you have to take it twice if the gun has an additional clip.
3.Why would any one ad SS mode in to a gun.

I can't answer 1 or 2, but I can answer 3.

One would add Single shot mode to a gun for covert uses, like with a silencer, or for using special kinds of ammunition. both require that the action not cycle, and that the casing stay in the chamber, the former so it doesn't make noise (and fall on the floor), the latter to prevent damage to the mechanism.