Questions about putting up a racing server

[T.L]

So.....I have had some thoughts about trying to put up a server
using certain mods but what is the policy around this? its not mine
mod, should I ask the ones behind it or how does this usually work?
Its going to be public and not any league, probably most practice for fun.

Another thing, I know that, atleast with steam, that it automatically
downloads mods and addons even if you dont have them at first,
which is really practical but this is a paid mod so is there any option
of disabling this auto-download so that only people that have the mod
in the first place gets to enter?