[Hectic]

Sidearm isnt really needed. Its not a bad thing to have but you will only really need it if your primary goes down (ie totally out of ammo or dead battery)
Your mp7 is small enough to fit in some holsters. And there are some holsters for it on the market but you dont really need a sidearm unless your a sniper.
Canadian tire has the crossman c11 (i think thats what its called) its a co2 non blowback that is a good cheap sidearm (one of the few good clearsoft guns) or you can browse around for a gbb if you really want a sidearm but id invest in a good primary and extra mags befor a pistol (unless you got a big budget then go nuts lol)