7.4v lipo is perfectly fine.
if you where going to a 11.1v then I would see issues.
As for the shop guy saying the cheap charger is fine.
it might be "OK" but the B6 is much better and honestly when using lipo its not something to cheap out on. Go back and get yourself the IMAX B6
"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."