re: whether or not BDSM can be a healthy dynamic, it really depends on how honest and aware you are of any emotions brought up by the ways you play, and your ability to deal with these things.
I like the body/mind fuck of kinky play, but combining this with poly relationship dynamics sometimes meant an extra layer of negotiation. For example, I noticed that my first partner became less interested in rough sex after I got together with a second partner who also enjoyed kink (we're more of the fluid/switching kind of folks, but here I'm thinking particularly of me subbing and them domming). After a lot of unpacking, we discovered that my first partner had assumed inferiority to my second partner in this department - the latter having more brute strength, being more passionately vicious etc. And when we (the two of us) would play with d/s or s/m dynamics, my first partner couldn't help feeling like I'd prefer it with my second partner and, not wanting to be a poor substitute, simply wouldn't be into it.
We both could have continued to superficially play like always, and not addressed our feelings. But talking was essential to keeping it real.
The same skills which help with poly in general (open, honest communication etc) would make BDSM dynamics work, I reckon. Good luck and have fun