Will grooves on seatpost cause rusting inside frame?
The best thing to do is apply a thin layer of grease to the inside of the seat tube and the outside of the post (only the part of the post inside the seat tube needs grease) and set the seat height according to what fits you.
The layer of grease will prevent the odd drip from causing rust.
If the bike is stored in a dry place after riding in the rain it will be able to dry out.
The bikes with seat posts rusted in that I've seen were the ones that were stored outside and never cared for.