I'm worried,occasionally, I find my cat nibbling on my dog's food. Will this cause any health problems? I try to prevent it, but he's sneaky!
Any answers will be gratefully appreciated!

While cats have specific dietary requirements, a few nibbles here and there is not a problem.
As long as his primary diet is a high quality cat food and he eats it well I wouldn't stress over the occasional sharing.

Here is a short and sweet publication on cat nutrition you may find helpful:
http://www.vetmed.vt.edu/vth/sa/clin/cp_handouts/Nutrition_Adult_Cat.pdf