My questions:
1) Can I import a WIFC key, i.e., compressed private key, beginning with "K" or "L" rather than WIF key beginning with "5" ? Does Bitcoin-QT accept both private key encoding formats -- i.e., compressed and non-compressed?
2) If I import the key from another client, should I be concerned that the key exists in two wallets?
Please note that I do not want to simply send funds from one client to another.
Thanks in advance for any advice.
1) Yes. It uses compressed keys itself now.
2) Yes that might cause confusion about where the funds from the key went. You also may want to learn about change addresses.
https://en.bitcoin.it/wiki/Change
If you are trying to spend funds in a paper wallet you want to make sure that it is not reused as a change address. So ideally don't import it into an existing wallet. Instead create a new wallet, and send all the funds to an address in your primary wallet.