These policies span across the whole network no matter if the plane is Midwest or Frontier branded. The airline is Frontier, so when an announcement is sent out that says Frontier airlines, it also is for the Midwest flights.
Here's the rule for the amount of pets you can carry on-board:
Make advance reservations as no more than ten pet containers (or two containers on Q400 aircraft operated by Lynx Aviation) will be accepted per flight and no more than one pet container may be carried per person.
So you can personally carry on one pet in a container and place it under the seat in front of you. No more than 10 carry on pets are allowed per flight expect for Lynx which is 2. So if you can find friends you can carry on more pets, but they would have to carry on the pet and store it for you.
Hope this helps.