In the event that a mod does't work, the first thing to try is moving it from the Steam Workshop folders to the corresponding subfolders in Documents, as described here:
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1309774090

That eliminates Steam as a possible source of error by giving the game itself full control over content.

Adding new modules (campaigns) to the game is normally conflict-free, because .mod files and their .hak files almost invariably have unique names, and cannot otherwise interfere with one another. So, modules can be added and removed in any order.

The problem tends to come with gameplay mods that use the override folder. There is no guarantee that they will be mutually compatible - they can even screw up modules, which are otherwise well-behaved.

As a rule-of-thumb, two mods that include a file with the same name won't be compatible. Otherwise, while conflict is always possible. the risk is usually low unless the mods include scripts (.ncs), especially scripts dealing with the same aspect of gameplay.

Introducing custom armours into existing modules is not impossible. If the author has provided a .uti file and .2da files, it may be as simple as dropping them into override with the model mdl and its textures, then using the debug command dm_spawnitem in game.

Sadly, this rarely happens. You can edit 2da files with text editor and make a .uti file in the toolset, but there is a learning curve that's probably not for everyone.