You can buy a pinion puller for 10 bucks. I got 1 and its a very good investment.

As for putting it on I bought a small C clamp.Set the pinion on the end of the shaft and with much fumbling and dropping the pinion youll finally get it pressed on there.

As for the pinion size if you look on Horizon Hobby site under hopups for the pro, they list the pinion needed.

Dont know the answer to the Q about your existing pinion size.