Being Secondary can be difficult at the best of times. There are periods of loneliness, doubt, insecurities, need for reassurance.
So, if you are already struggling with what it would mean to be Secondary, then I would strongly advise focusing your efforts and time on developing yourself and seeking the form of relationship that you truely desire.
However, attitude is everything! You have already determined that Monogamy is not for you. So if you seek comfort, part time companionship, friendship, by all means, date someone casually , as Secondary if need be, but keep a very open , clear mind about finding your own Primary.