SleepyDiary,The chart should simply be interpreted based on what is there, not with any assumption that it 'should' be another way. So with Gemini over the cusp of the 5th it (and Mercury) colors the general expression of those matters. So what you've described is that Gemini IS on that cusp, so it doesn't matter if Cancer isn't.
You might find some connections with children or romantic interests or even business partners who have placements in Gemini.
No, it doesn't mean Cancerian people won't have planets in your houses.
It doesn't matter when a sign rules two houses. What matters in such charts (assuming they are accurate on the Asc) is the house pair that is expanded so that they entire engulf a sign pair. That is called in interception and it widens those houses, creating at least a bit of extra emphasis in those matters. So for the 1st/7th it would be relationships/teamwork; for the 2nd/8th it would be income/money; and so on...
------------------
Soul Stars Astrology - by the declinations guy
Declinations -
because the planets move north and south of the celestial equator