Personally I use 21 gauge 1 1/2 pins. I do 2cc injections eod right now you should be fine putting the deca and test in the same barrel. I would do a rotation tho like right glute, right quad, left quad, left glute, and then repeat. That will give the sites time to heal so you don't build up scar tissue
And if you want the oil to flow quicker out of the pin hold the barrel of the syringe under running hot water right before you inject it will flow much easier