First of all, I love ALiVE and am really enjoying it.

Okay so I have looked online and haven't been able to find a solution to my problem.

I play with ACE3 and mostly just play by myself with missions I have made. Because I use ACE3 the medic isn't exactly great controlled by AI so I normally just make everyone in my squad playable and switch between as I need them. However if I call in reinforcements the added players are not playable. Is there a way that I can changed that in anyway?

Depends what you want.
If you want your medic do his work with ACE 3 there are a script to force unit being a ACE 3 medic.
If you want to switch units look into your description.ext

disabledAI Removes all playable units which do not have a human player. (MP) Note: Disabling all the AI units will prevent JIP into playable units disabledAI = false/true

And look into this post:
Unit Switching

Thanks, I think that will get me what I want probably on both accounts.