Felicia,
Yes, I use one or the other. And, as Steve said, modified XFZYT is usually my first choice. I mentioned the XFHMY because you had said the mass was quite large and a lot of times these don't heal well post-op due to residual disease and larger tumors can often be quite erythematous and inflamed so I didn't know your patient's particular situation. If the surgery site looks good, I would go with XFZYT. Don't be afraid to use a generous dose if you are concerned about a more aggressive tumor based on rapid growth rate or lymph node metastasis. Some of these grade II tumors can behave more like grade III tumors, others are more benign in behavior.
Michigan State University has a "mast cell tumor proliferation panel" that can help predict behavior of grade II tumors if these owners are interested in being more aggressive with therapy. In cases that have high proliferation indices or c-kit mutation, they may elect to use chemotherapy in addition to herbs. I'd be happy to help you with interpretation of those results if they would consider chemotherapy in a more aggressive tumor. I have used XFZYT with chemotherapy. They seem to tolerate it well, although I have had a couple severely blood deficient dogs that have become quite neutropenic at normal chemotherapy doses. So I tend to be more cautious in those patients and use Vinblastine rather than CCNU/Lomustine.
Erin