Regulus watched Sirius and the Marauders target Snape and knew the strategy.  If Snape was focused on his Gryffindor peers, he wouldn’t be experimenting on firsties.  Regulus wasn’t certain it would work.  Snape would want confirmation that new spells would work before trying them on his arch-nemesis.  The embarrassing possibility of a new spell fizzling on his enemies would drive him to more practice.