yes a 90 hp skid steer is a pretty large one(mine is only 70 hp)
but a 100 acres is a BIG piece of property to clear with a skis steer of ANY HP
if all things are mostly smaller, I personally would rent a dozer, push into piles, and burn as weather allows
you will save that skid steer a TON of abuse and hard work, and get things done in a way way faster method!
, as if your planning to do a food plot or?? with a brush mulcher, your still leaving behind a TON of roots that will be a major problem when planting anything down the road
requiring a excavator to come in and dig em out, where, you MIGHT get lucky with a dozer lifting and pulling roots out as you go, saving you a lot of work down the road!, all the more so if you can get a root digger for the blade(or a decen ripper behind it)
THEN if you desire to BUY an attachment for your skid steer buy a grapple bucket, will come in handy a lot more IMO and be a better investment
as what a mulcher is going to costs, for a good one, you can rent a dozer for a few days and maybe even get the whole job done and not put of ounce of wear and tear on your skid steer a decent dozer will have a LOT more weight behind it than your skid steer
and if your NOT great at running equipment, havinga hired guy that is is m,any tims worth the $$$, as they can do ina day maybe 2-3 times what a rookie can as they are just more efficient than others!
MOST can walk on a site and give a full on price of what the WHOLE job will run you
I like doing work myself(maybe too much at times, get carried away and don;t want to stop once I get started) so I get this part,
but in the bigger picture, that skid steer was NOT cheap to buy, why abuse it and take 5 times longer or more and not really save any $$$
just food for thought here