I was wondering about the 20 watt JCA20H. If I bought this would I be able to hook it up to a 4x12 cab and be loud enough for most gigs after running it through the P.A?
As long as the head and the cab are the same Ohm rating, yes. And even if they're not, it's not that big of a deal.
Also could I get a more metal sound out of this amp with a pedal?
I have a Boss DS-1 and A Boss MT-2 Distortion pedal and I could get an overdrive pedal if needed?