The idea that you'd expect a head count before making a decision made me chuckle; reality is messier than that. As for negotiating with the enemy, that requires an enemy amenable to negotiation in the first place, which is an idea that would have applied when the Rules of War were drawn up, but (unfortunatel) doesn't apply any more.
Sarcasm on the headcount, as it's a nonsense question every situation is different you can't set a number threshold at which you suddenly shoot at medics. On the subject of lines of communication isn't it the western powers who are refusing to talk ?.
Short term diplomacy could involve yelling though a loudhailer "abandon the hospital or we return fire" then returning fire after a suitable delay, this would be acceptable under the Geneva convention if the hospital was in use as a fire position (and only if it was in use as a fire position and only if the demand had been made). Abandonment of the Geneva conventions is not a good thing for mankind and could have absolutely awful unforeseen consequences in the future, luckily most nations still respect them.
A particular instance tells us nothing about wider morality, not that I am not defending this specific attack since I have no information on the matter.
The Geneva conventions were written in response to a wider demand for morality. The MSF are not renowned for propaganda unlike every news source on every side.
So your answer to my question boils down to, "No, there is no conceivable number of casualties that would make me risk a single medic".
There are no circumstances where I would sanction specifically targeting (real) medics/hospitals with overwhelming damage from the air. There are alternatives they should be explored (ie contain the area and get ground forces in to confirm the situation, then deal in a way that minimizes the risk to medics/innocents).
If the Taliban were inside the hospital using the building as a fire position (and it's a really big if) I would support the use of ground troops to kill or capture them (following a demand to withdraw being given see above). I would expect those troops to be very cautious of shooting non-combatants.
It's not compulsory to flatten any building you think the enemy might be in then shrug and go "hey but we're the good guys" when it goes wrong. This dismissive approach and attitude to casualties among non-westerners goes a long way to explaining why western intervention has destabilized the middle east.