I was in a very similar situation about 13 years ago. Short version: My alma mater had 5 sororities, 4 NPC and 1 NPHC. During the year before I enrolled, seven Jewish women got together and chose to form their own sorority. Specifically, they wanted to bring AEPhi to campus. They petitioned Panhel, but the 4th NPC sorority had just colonized 2 years earlier, so Panhel denied the expansion but invited the women to form their own local sorority - which they did. The following year, the sorority had its first new pledges, including me. The year after that, we chose to pursue membership in AEPhi again. This time, Panhel was agreeable, and a match was made.
When expansion happens on a campus where there are several NPC sorority chapters and a local college Panhellenic in place, one of the following can happen:
1. Panhel can announce that the campus is open for expansion. All NPC sororities not already represented there are eligible to present. Then, Panhel chooses an NPC sorority. That sorority then sends representatives to campus to choose the founding members.
2. If there is a local sorority, that local can petition Panhel to become part of an NPC sorority. If Panhel agrees, information is sent off to all NPC sororities not currently represented at that campus. Interested sororities send representatives to meet the current sisters of the local. From there, an interested sorority can accept the current sisters lock, stock, and barrel, and/or can conduct interviews to identify additional colonizing sisters. Alumnae of the local are usually invited to participate in initiation.
A local sorority can express an interest in a specific NPC sorority. My sorority expressed a particular interest in AEPhi. We still received material from several other NPCs, but AEPhi's values and goals aligned closely with ours. You do not necessarily have to go with a dormant chapter; if you feel that ABC (who never had a chapter at your school) is a better match than XYZ (who has a dormant chapter), think about ABC. But it's great that you're considering dormant chapters. My alma mater has no dormant chapters.
Good luck.