One thing I'd like to do is make it so that an item type can use the proficiency ability from another of its type, for example having a pistol utilize the arquebus proficiency modal and vice-versa. How would you go about this?

I can add the StatusEffect IDs of the pistol's wp prof to the Arquebus's wp prof, and vice versa, but that means that when you toggle the modal you'll get all the effects at once. Or give shields access to all 3 of the shield abilities.

How would you do this? I tried looking in the items list some parameter that says what modals the weapon has access to but didn't find one; the LaunchOnEquip IDs are all related to item properties and not modals.

Ideally what I want to do is: you equip a shield, you take the shield modals and regardless of small/med/large the equipped shield grants access to any proficiency you've taken.

I.E. you take all 3, you have all 3 regardless of the shield. I can't figure out how to accomplish this via IDs.

You probably need to edit the Items.gamedata for each Item you want.

I didn't test it, but here is an example of what should work:

 

There is a field named ""ProficientAbilityID" which lists all modals available for that specific item.

 

For example, the Watcher Sword has "ProficientAbilityID": "e56c9809-5046-48a8-9334-491b053c79a7".

If you want to add another modal, then you need to add another ID after that one.

 

For exemple: 

"ProficientAbilityID": "e56c9809-5046-48a8-9334-491b053c79a7", "111aae3f-ba8f-4cf9-a766-58e7d1e8e8ac"

 

Now the Watcher Sword has the Half-Sword Modal and the Engaging Thrust Modal if you possess both Half-Sword and Spear profiencies.