Switching from live mice to frozen thawed rats?

[Bogertophis]

... Was also wondering if it would be rude to ask the seller about if they’ve tried with rats as some of their snakes are eating FT rats, but don’t want to waste the sellers timeZ

It's not at ALL "rude" to ask about that- it's essential information for you, the buyer, for best results for everyone, especially for the snake. You also have to hope they're truthful though- sadly not all are, & will just tell you what you want to hear to make a sale. If you can ask in a neutral way (so they cannot tell what you'd prefer to feed the snake) you might get the best info.