Here are official pictures issued by Naloger developement, showing that a new tab "GTA Mixed" tab will be added to skins, vehicles & weapons. You'll be able to put GTA Mixed skins, etc to your missions in GTA:SA, or if you have GTA:SA with GTA Mixed - you will be able to choose it more easily without changing any files.
[UPDATE] - Clarification, and sorry for misunderstanding: only GTA Mixed users will be able to see the tab, check will be implemented that if a user don't have GTA Mixed he'll not be able to play mission with those cars / vehicles in it. Basically in order for other people to play your mission with those cars, weapons, etc - they should also have GTA Mixed installed.
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Clarification - only GTA Mixed users will be able to see the tab, check will be implemented that if a user don't have GTA Mixed he'll not be able to play mission with those cars / vehicles in it. Basically in order for other people to play your mission with those cars - they should also have GTA Mixed installed. Sorry for misunderstanding.