Knoppix is made of the mix, so yes doing a Debian install will get you the mix. About the only way to not be bothered by it would be to do the "poor man's install". You still have the mix, but since you can't install anything else you don't really have a problem from that. But your original statement of the problem is not completely correct. To say that the main reason for not installing Knoppix is the mix and match problem ignores all of the problems of things that work fine from the CD but often stop working after a hard disk install, like networking.

Have you ever tried an install of Debian Etch? I'm hard pressed to find any reason to install Knoppix rather than Debian, and can give a number of reasons why Debian is a better choice. If you have a good reason for fighting with a Knoppix install over Debian that would be interesting, but most of the people who ask about Knoppix installs have never even tried to install Debian. That strikes me as, well, lets be kind and say "foolish".