First of all, it is better to stay on test if you're adamant on taking time off opposed to doing a lot of PCT's.

600 deca and 600 test is quite a lot for someone who is combining oils for the first time (I did 250 test / 375 tren as first oil combination), it should however provide you with some serious gains. 600 deca and 300 test would also be great.

For injections, it doesn't really matter. If your ass can handle 2ml at a time, it's fine. If it can't, shoot 1ml in the ass and 1ml in the leg.
Pro tip: learn to rotate spots early on because scar tissue is a motherfucker