If you got a reasonable response from the manager of the shop and feel confident that he will do what he's said to treat the infested snakes (and I assume check his other stock), then only you know if you made the right call by supporting the shop with your business.
You know what you're looking for with the new snakes, so keep them quarantined and keep vigilant.

Regards feeding - small smelt cut into pieces slip down very easily, pinkies cut into small pieces (a bit more messy) or obviously fish fillets can be cut into the appropriate size.