Lymphoma is actually one of the rarer types of malignancy in the parotid gland, much more common is one based in the epithelial cells, and it is usually low grade, slow to spread, and has a high survival rate.
I don't know about any corelation between the presentation of lymphoma and the state of advancement of SjS, in fact there are cases where the SjS is diagnosed incidentally after the lymphoma, and is not severe enough to have sent the patient to the doctor for diagnosis until the lymphoma symptoms presented.
At this point, a rheumatologist wouldn't be my first choice, I'd prefer to be in the care of an oncologist (cancer specialist).