Can I cook lentils in a rice cooker? Would I need to adjust my lentils/water ratio?
Almost certainly: lots of recipes online.
I'd go with the 'porridge' or 'brown rice setting' if your cooker has it. Concensus on water ratio seems to be about same as for rice.
Here's a question which lists More things you can cook in a rice cooker.
With a quality cooker, you can also make baked beans and tasty onion soup.