I think it matters what type of singleton we are talking about.For example, my Jupiter is a hemispheric singleton, not in terms of aspects. being the apex of the chart, so to speak, any conjunction there would have a specific value, related to its apex position.
PM's fire singleton is also an apex planet. His chart is basically made of 2 Kites, one T/square and the angles Cross with Sun/Saturn/Pluto. One of the Kite apexes is Neptune, the fire singleton. A conjunction there will highlight the "mission" of the singleton.
So, I'm thinking case-by-case value again.
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I seem to have loved you in numberless forms...