Yes, do go slowly at first. Kale is a brassica, like cabbage, and can be "gassy". If you ease it in slowly as only one green of many, their systems will adapt to handle it. If you feed to much at first you may have problems.
I feed cabbage from late fall when the weeds are dwindling through until it gets too expensive in late winter. I start each year with just a few shreds added to their usual weed mix and work it up from there. Doing it this way, I have never had a problem.