They did, during the great shism in halo 2 and halo 3.
There is a shipmaster who returned to the previously glassed colony of Kholo who decided to save a human that was shot by jackals.
Yes, being the honor-driven race they were, if they felt an enemy soldier fought valiently and with honor, they might have that soldier fight along side them. This was the case with some Grunts diring the Grunt uprising. Grunts whom the Elites believed fought with honor were given a place in usualy exclusivly Elite military roles.
When an elite finds himself in a life or death battle then realises he is about to get blown up- he will hold you lovingly and grab your ass.