Example, slightly editing some stuff in the pickup.dae file. Can I just upload the whole Dae file with everything in it, or will I have to remove everything except the edited items and/or rename the dae file? I'm new to modding and am trying to make what I have (a retextured gauge cluster + moved needles around) a selectable part instead of it just replace the part until the mod is disabled. Also I have no idea how mod slots work (as in where they go, what I have to write)
If you want it to be selectable apart from the original parts then yes better make a separate dae containing only the parts you changed.
I suggest taking a look at cars with multiple gauge skins ( like metric, imperial, ... ) . You're going to have to make a slot for your parts.