I'm currently AFB but I remember discussions in the past about the fact that the book basically says that med kits have stimpacks in them but never really specify how many. My solution is this: the med kit has a special chemical making plant that gives the equivalent of one stim per player per day. To put it in d20 terns, the med kit has 3 "charges" (# of players) per day that can be used on anybody in the party. How game breaking of a mechanic is this?

Its not gamebreaking at all. Technically the book says 1 per 'scene' which I interpret as ENCOUNTER.

Here is the houserule/clarification that I gave the Doctor in my game. Because for the most part in combat he is patching up injured players, I am lenient regarding it not requiring 2 hands or having him stow weapon in his other hand to use the Kit.

Medpac will be known as MedKIT. Medkits allow 2 stim uses per encounter and cannot be separated from the Medkit

We've gone thru 5 games so far and think he has only used 1 stim from the medkit so far. If you arent trained in Medicine there is not much point to carrying around a 2 encumbrance MedKit for 1 stim use per encounter. It would take 16 uses to pay for itself.

I think it's unlimited in the scope of an adventure unless it needs to be limited, or you roll a Dispair or a couple of Threats like it works with weapon ammunition. Basically you can use it once per scene per PC (usually 3-6 PCs) and that it has enough charges to cover this until you can re-charge it between adventures.

If you need it to be limited, say you crash land on an uncharted planet, then you can place a hard limit. I'd just go with it runs out when you roll enough Threats or Despair.