Oh! Definitely! Mars-Libra connections have never failed me! All my partners have had at least one Mars or Venus in an air sign. It is the tell for me whether or not the attraction is mutual. The typical scenario for me is a Gemini Venus aspecting my Aries Mars or Gemini Mars aspecting my Libra VenusThe most common combination I've dated is Venus in Scorpio falling into my 7th house conjuncting my Sun and Merc and their Gemini or Libra Mars aspecting my Libra Venus. And usually their Sun, Moon, Merc are water or earth to match my water/earth makeup. And usually this leaves my Aries Mars out to dry but it's never too bad.
I'm glad I brought this to light! It's the most basic but insightful thing I found in synastry.
Now for the cancer person, you could totally do the composite but you'll have to ignore the combined stuff that involve time I think. So ignore houses, ascendant, ic, mc, dsc, and whatever else. But your planets and their aspects are usually fair game. The moon can be iffy but only in an extreme case like you were born in the US and he was born in Australia :P
But! A big chunk of the story is lost with the houses in composite. And the combined ASC is fun because it lets you know how people see you as a team.