Slickmod requires the host to be using the same version of slickmod as you, just like AndroidXP said. But smokemod is a mod that can be used online+offline on any server. It only effects visual things on your computer, so it will not do anything to the server.
None.
Perhaps sounds activated at certain Revs, because Gunn is right, you won't be able to hear any sounds when your engine is roaring. But being a drift and slacker usually I park my car next to some one else's (Out of the way and off the track thank you) at least one every time I'm in a server.