does size matter,,,pellets/./

ok,so im about to make my first order for 30 cal pellets ,i have 2 brand new 30 cal rifles on the way[ fx impact and a hm1000x] the pellets and slugs i like, vary in sizes from 30 cal to 303 to 308 ,,do i need to stick to straight 30 cal. will 303 and 308 fit,will i loose accurasey ,will they mess up the barrel, do the bigger sizes matter at all.. thankz the rookie