Whew, ok, let me try this methodically:
No, this assumption is not valid, at least not in the case of KDE2 and KDE3. KDE3 seems to go into the /opt directory, making it completely separate from KDE2.
I doubt it would matter if you uninstalled them, as long as you do have the newer packages installed. I didn't uninstall them though, so I am simply guessing that it would not "hurt".
Yes you can resize, use a program called Parted
Good luck, and if those answers don't help, please let us know.