My situation is a little like that, although it's still in the hypothetical stage.
I'm my own primary and am happily single-by-choice. I'd like to have two or three lover-friends who do not live with me. I suppose they would be secondaries to my own self-primary status, but they will not be hierarchical to each other.
I have only one lover-friend right now. He is single but is looking for a primary partner / life partner / marriage-and-kids partner. If he finds one while he is still seeing me, she will be his primary and I will be something like his secondary (which sounds great to me).
But I won't necessarily consider him my secondary, if that makes sense.
I haven't yet dated a married/partnered poly person, but if I do, I expect to feel comfortable with hierarchy and being a secondary (or being one of several secondaries). In fact, the appeal of not being someone's primary is why I seek out poly men in the first place.
However, from my point of view, my lovers (regardless of whether they have their own primaries) will not be ranked in hierarchical relations to each other.
I don't see why that can't work.
Single, straight, female, solo, non-monogamous.