Headmasters IS canon (oh how I hate it, but it is)
Headmasters IS NOT in the US G1 Toon continuity.

so remember children, canon =/= continuity. Transformers has alot of continuity families, all are canon, but separate universes.

Headmasters =/= US G1 Toon just as much as Beast Wars =/= Bayformers. BUT ALL ARE VALID!