It isn't a good idea, especially with the Red Hat RPMs. Different distros put things in different places, and you may break something using packages from another distro. Also Red Hat 7 packages are completely incompatible with other distros because of the mutant GCC they used.
As for the Mandrake packages, here's what the guy formerly in charge of packaging KDE2 for Mandrake said:
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Hello,
I can no longer speak for Mandrake as I am no longer working for the company, but here is my guess: There was probably a slight confusion over who is responsible for making the new RPM's. I had been creating all RPM's for the 7.2 KDE updates until about 2 weeks ago. It takes time to reassign responsibilities. I have had some email with people who know what is going on and they are working on building rpm's for kde 2.1. You have not been forgotten about.
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Me, I just compiled KDE 2.1 from source.