Is it safe to do this when working out with dumbells?

i am 14 and i just got dumbells, i was wondering if it was safe to do 25 curls then 25 hammer curls (i think that is what they are called, they are like curls but u hold the dumbells with ur palms facing inwards) without any kind of resting inbetween, is that safe or is it better to do 10 then rest a bit then 10 again etc...and if i was to do that how long do i rest?