Submission and domination

I'll answer the second question of yours first because it is the easiest. No not always. Sometimes you come across dominant feedees and submissive feeders. Typically though, the feedee is usually submissive to some degree and the feeder dominating to some degree.

Feederism is mixed with BDSM for a few reasons. The first being as explained above, there is a dominant party and a submissive party, usually. The feeder also urges, drives, forces, the feedee to his or her eating limits much like a typical dominant would use something like pain to drive a submissive over the edge and into ecstacy. Additionally, there is the aspect of humiliation with this particular fetish. Some submissives like being called out for being fat, forced to wear tighter clothes as they get bigger so that everybody can see their gains, thus extending the humiliation, and control outside of the bedroom. Lastly, there are some instances where bondage is what turns people on. Typically the submissive is tied up to some degree, not in crazy upside down suspensions (physics isn't that forgiving), but bound and then fed, sometimes why more than they ever thought could fit inside them.

How to incorporate BDSM with being a pig? Hmmmm...that's a bit tough because it depends on the submissive's tastes and the dominant's skills and resources. Many of the situations described above can play into what you can and can't do. Force feeding is probably the be all in this area. It is bondage and physically the dom is forcing the sub to do something that they cannot resist. You mix something like toys, machines, possibly hot wax along with that but you have to be careful and go slow like all things BDMS. Technically, force feeding can be seen as a form of erotic asphyxiation is the feeder is really fast and forceful. There things like tight clothes or forcing a submissive to workout as forms of humiliation all the while commenting on how fat they are. Certain bondage and rope tying works just as well with a piggie than if he or she were smaller. There is hot wax and electricity as well that you can use. Just be careful, because in my experience as a piggie gets bigger the stretched skin gets more sensitive so it doesn't take a whole lot to get the point across. Lastly, the rewards are different. You don't always need to reward the piggie with a reduction of pain or a pleasureable orgasm. You now have another option, and that is to feed them because that probably turns most piggies on more than anything else. So you can use it as a very tempting reward.

Anyways, I hope that helps.