No, sorry.
A composite is made up of the midpoints of two planets in the natals.
A midpoint can represent two planets that are in trine, square, opposition, etc.
Both easy and difficult aspects.
So if there are two mars in the two natals in square, but their midpoint touches the midpoint, lets say between the two Jupiters, we would have Jupiter conjunct mars in the composite, which shows that in the relationship the negative aspects of the mars square will be mitigated by their contact with Jupiter. But the mars square still exists and manifests its energy in a harsh way between the two persons.
Not sure if this is a clear explanation, let me know.
Basically you cannot substitute one for the other.
About the difference between the sun/moon conjunction in synastry, which is a strong and positive contact, and the same one in the composite, follows the example I gave above. The composite sun/moon conjunction could be actually made up of unrelated sun/moon contacts between the natals, even in difficult aspect (moon square moon, sun opposing sun, etc), but they work out as a conjunction in the composite. Which shows that in the relationship the two energies can express themselves in a positive way.
Thanks for this interesting question!