I recently purchased a Wallace Heaton 9x12 plate camera, which came with 6 plate holders (and a pack holder). When I got hold of some 9x12 sheet film I then found it that it was too big to fit in the plate holders and had to cut it down to size.
I used a roller cutter, with some pieces of card taped down to provide the correct dimensions. It wasn't difficult, although it was a pain as I had to unload the sheet film, cut it in the roller cutter and load up the plate holders using a changing bag - which also got rather humid!
I have just exposed the film (Fomapan 100) and I'll let you know how it turn out when it's developed.