At an early point medics healed me for credits but I see this is not the point any more. Since I have all kinds of medical items, can medics use them in any way? Or are they only used for surgery/implants?

Only PCs can use med items.

They don't currently have a use I think - although maybe used for cybernetics, not sure.

Adding the ability for NPCs to use medical items is one of the things on my list.

Thank you for the work you are doing for the Combine, Mikel!