I don't think they necessarily have to be the sign on the 7th house cusp. But the overall energy of the individual is usually indicative of that.
The stronger the representation the person is of your 7th house cusp,the more attracted you can be to them.
-Are the partners you are drawn to often "innocent-looking" or not very grounded in the here and now?
-Do they always seem to need "help" in sorting things out or come to you for clarity of issues?
-Are they often sick/impaired or "going through something difficult" when you meet them?
-Are they usually artist/musicians? Or devotees of some religion or philosophy.
-Are they recluse? Prone to wanting to be alone?
-Are they just too "chilled" for your liking at times. Too easy -going and breezy?
Look at all the Pisces traits and see if they apply to the people that you attract consistently. This is the real meaning of the 7th house cusp IME.
Usually though,I notice that Moon in a certain sign is very sensitive to my 7th house cusp; either they have a sign that is exactly on my 7th house cusp as their Moon.
Or they have aspects that "bring out" the energy of the Moon to be consistent with my 7th house cusp sign.
Those are the ones that I am MOST attracted to.