“if there is a way to mount a shotgun mic to my old Sony 8 camcorder”
Yes. You’ll need a camera bracket, something like this one: https://www.bhphotovideo.com/c/product/133276-REG/Bescor_VB50_VB_50_Universal_Shoe_Mount.html
and a microphone mount to fit into the shoe, like this one:
https://www.bhphotovideo.com/c/product/225809-REG/Azden_SMH1_SMH_1_Shock_Mount_for.html
“or rig it so I can carry and use it and my camcorder at the same time”
Yes, with some caveats. You can get a boom pole or a handle for the mic…but it’s better to have someone else hold it. Remember that the key to good audio is to get the mic within 2 feet or so of the sound source. Your camera, and you, are probably not in the best position to capture the audio. I’m sure you can find a volunteer to hold the mic for free.
“can I hook up the mic to a minidisc recorder if I want to record sound separately?”
Yes, but try it out with the MD before you buy it. The MD recorder might not supply it with enough power. In that case, one of the mics with its own power module would be required.
Regards,
Doug Graham