See the answer below.
Yes. For example, dodecagons, squares and triangles.
Semi-regular tessellation is using multiple copies of a few (more than one) basic shapes to cover a plane space without gaps or overlaps.
They can be but need not be.
must all edges of semiregular polyhedron be the same length
Yes it is a tessellation.
Non-visible tessellation or non-existent tessellation, perhaps.
How to Make a Tessellation?
A regular tessellation or semi-regular tessellation or none.