thanks for your very useful Answers/information @Reyser.

I'm doing a combined wheels/rims addon pakcage, does this line effect any visual or non visual(game physics) in game :
<rimRadius value="0.348500"/>
what i mean is can i make all types of wheels have the same value (to speed up the process of editing carcols) ? i found some offroad wheels have different values, and did my test change it but didn't notice any changes.

So I did the first pack successfully, and everything was working fine up to when I decide to make my 2nd pack which contain another 10 addons cars. the game load normally but No I can't spawn any of the 2nd pack cars with error msg " Invalid model" , i spent a couple of hours trying to figure out the issue and then i decided to change this line in setup2.xml in the 2nd pack.

<order value="11" />
change it from 9 (like the 1st pack ) to 11 , and it worked I can spawn 2nd pack cars.

I have no idea what is this "order value" for, I've seen other dlc shares the same value with no issues, if someone have any idea what is this value for please explain and if someone have the same issue you might consider this solution too.

I found this very useful and easy to understand with all questions & answers from other users. not all mods have the same structure, some you might have to add/edit and combine more *.meta files and add it to content.xml like the *contentunlock.meta , etc.
I combined 10 addons, some of them have tune parts, additional wheels..etc and it's working fine up to now. i spent many hours to setup and compare between the first 4 mods but after that it's more easy/clear and you can add more and more in just few minutes.