Yes--too many fleet types are hurting you. If you didn't choose the most popular fleet types in the game (737 and 757) you'd have a much tighter fleet commonality as aircraft would be more available.
You should be oversupplying every route you're flying before moving on to new routes for a few reasons. Number one is you want to discourage competitors from competing with you on the route. Number two is that you cannot capture all the demand with only 100% of the seats. For example, if you have 1000 pax demand and fly 5x daily with 200 pax aircraft, you'd have to have 100% load factors to carry them all. We all know this is not possible. Therefore, in order to capture as much demand as possible, you need to fly 6x daily for 1200 pax and hope to get 80%-85% load factors to capture the entire demand. Also, if you're flying a 200pax aircraft 6x daily and your competition is flying 50pax aircraft 6x daily, their small aircraft is going to fly full whereas your big one will be half empty. Essentially, there is no excuse for not having 50%+ marketshare on any route you fly (or at least #1 marketshare if you are competing against more than one airline on a route).