i dont see any problems with that combination. I have a giraffe cat that is with several synos. A giraffe is like a niger. but my situation is reversed. The giraffe cat is larger than the synos by far.
The only problem i for see is the Frontosus being a little nippy, just because of its size and the competition. But im not very familiar with this species. other than that i dont think you'll have any trouble.