What do you want to tell me? There is no vanilla definition file of Ruse because it's basically not included in the game or a DLC.
Yes, but that's not the problem. If you use both of them, ProMods' Romania would be replaced by Andru's Romania, also the defs.and also.. as you can see in my signature.. i`m using it w/promods w/o any problems.
So if Davyddin (or I, because I also do a city names mod) would choose the definition file from Andru, this would be possible in your case. But if then somebody uses only ProMods, the game would crash at the truck dealer.
If Davyddin would choose the ProMods file, it won't be compatible when somebody plays vanilla map plus RoExtended.
Unfortunally, both SII files are named identic ("ruse.sii") so we can't include both at the same time. Some mappers use unique names that don't conflict with other mods - Project Balkans for example used "ljubljana2.sii" and so on.
But in this case, it's simply not possible without creating a seperate add-on. I will do one for my mod, but Davyddin doesn't want it (which I totally understand).