You are using Audacity 1.2.6?
Don't paste into the same track. As you have worked out, they are at different sample rates, and if you have them in the same track any that do not match the sample rate of the track will play at the wrong speed.
Use "Import" to get the audio files into Audacity, and keep the different audio clips in different tracks. It is OK to cut and paste a clip within the same track (for example if you are using the same intro several times, you can just make duplicate copies in the same track. Use the "Time Shift Tool" (double headed arrow) to slide audio clips left/right.
When you Export your finished show, all of the tracks will be mixed together into a single stereo file (or "mono" if ALL of the audio is mono).
You would be better to use Audacity 1.3.7, it is a lot better now than the old 1.2.6 version.