I think you will regret lowering your platens right now. It sounds like your current OC is correct for T-shirts. If that is so then you should leave everything the way it is. Just use shims under the screens for thicker material, yes a pain right now to reset the job but it will be worth it. If you lower your OC right now to accomadate the fleece then you will have to raise the OC again once you go back to tees.
Your screens are not to tight, the argument presented that your choppers can not handle the higher tension is false because higher tension requires less squeegee pressure because you will use less OC.
From the sounds of your problems it seems obvious you have not enough or zero OC on fleece and also it sounds like way to much squeegee pressure. Get some OC off the fleece then start playing around with the squeegee pressure till you get desired results.