Will the old version of Airport Utility still be usable if I upgrade to 6.0? I would like to know if, once I upgrade Airport Utility to version 6.0, the old version I had previously will disappear, and I want to know if I can use both of them (old and new) on the same Mac.
Yes, the old version will be removed when you download the new one.

Yes, you can download the old version and use both, side by side (according to this TUAW article).