Honest question, what's likely to happen if you mix plastics? I understand how if melting points are different you might get odd results, and I'm guessing maybe very different materials won't bond at all leading to cracks in the final product. But if you doing something like they show in these videos, heating it to <200 degrees and just pressing the result into sheets or blocks that can be cut with hand tools, or doing very simple injection molding, can't you get away with a lot that might not be acceptable in an automated, professional setting?

Note that I'm thinking entirely in terms of what I might do myself in my workshop, not what'd scale to a city-wide or country-wide recycling system.