wait the petco was feeding it crickets? how big is your new bp? i assume it is quarantine from your other bp. it depends on how big the new bp is for what you should feed it. 10-15% of your snake's body weight is the best way to judge your snakes size meals.
maybe its me but if the petco store employees told me they were feeding their bps crickets i would leave and never go back to that store ever, and report them.
Tom