Manpower units?

Every unit has a purpose some are more useful than others, utility works to tow trailers and counts as a heavy rescue, fly cars have als capability and will treat a patient before an ambulance can get on scene, they also work to assist BLS ambulances to transport ALS patient

To answer the first question if you send a ultity and fmy car the mission /paitent will be prosessed faster

I've had some timers go down faster when sending an extra engine or 2, on fire calls.

The answer is yes, provided they are a vehicle required by the mission it will reduce the time it takes to complete. This is why i always laugh when people say to cancel your units A) there's no such thing as over resourcing a mission and B) you're potentially delaying its completion and holding up peoples resources for longer than necessary.

I have found that in some cases all you need on the mission is a fly car as it treats and releases.... some of the minor BLS calls

I always send double the amount of required engines to all missions just to get them done faster.