Yes it will reduce the likelihood of medical calls, but only if you are replacing your existing ambulance stations with fire/ambulance combos - otherwise you'll still get calls from the ambulance stations you have already anyway.
- If an ambulance station is chosen randomly by the system to spawn the next mission then it can only generate a medical call.
- If a combo fire/ambulance station is chosen randomly by the system to spawn the next mission then it can select randomly from both fire and medical possible calls.
Therefore blended stations reduce the overall percentage of medical calls (again, assuming you are deleting existing ambulance stations and extending existing fire stations to house ambulances).