The "extra" proficiencies

[Ardanis]

However, it would be nice if scs casters could deal with my created spells. I take it that they could as long as I 'tag' them with a proficiency that is associated with a similar--in the right way--spell, they--ie SCS, and even ee mages--can deal with them

 

If you mean the AI recognizing you custom spell protections and using Breach / Spellstrike to take them down in the same way it deals with vanilla Spell Deflection, then yes, all you need is to flag your spells accordingly with DS stats.

 

If there's a free proficiency right now, can you guys leave just one proficiency free for use by something other than DS?

 

I'm pretty sure it is free in general, and most certainly not used by DS. It's possible there's a mod out there that makes some use of it (in whatever capacity), but I have no recollections of such.

 

By the way, if you need a custom weapon proficiency, you can also re-purpose 108 for that - this is how Wilson's natural attacks work in BG2EE, and since those are unmovable Wilson-only weapons, you should be safe as long as you don't cast magic weapons on him. Even then, the worst case scenario is him losing / gaining some prof points for the duration.