Chance of doing P Coy with 75 - slim to fcuk all. The only RE unit that routinely do P Coy is 299 Para Sqn in Wakefield.
Chance of doing AACC with 75 - slim to fcuk all. The only RE unit that routinely do AACC is 131 Cdo Sqn in Plymouth/Birmingham/Bath/London.
You can volunteer to go on tour with 75 whenever you please but when you actually go depends when they are next required to provide personnel for deployment, speak to them to find out when this is. You will not be able to deploy until you have completed your trade training, you will also not deploy whilst you are under 18.