Joe, unfortunately it would be anyone's guess what you may need (guessing can be costly and dangerous) with out more information on things such as M_D mentioned.
What are you calling sheet?
One method is contact cement on a sacrificial board under the sheet then heat and remove, use clamps, screws it just needs to be secure.
Since the trimmer probably has a .250 collet use a .250 cutter, it is hard to go wrong with carbide and the cost of these is very competitive now for standard carbide cutters, advance geometry or special coating can jack the price up a bunch but I doubt you will need that at this time. I buy most of my cutters from this guy.
http://www.american-carbide.com/default.aspx