Is it OK to use Malwarebytes alongside Spybot on a PC?

Sure.

Yes you can run them both, but it is not recommended to run them both at one time. Keep one running at all times and when your ready to scan alternate which is running, Some of these programs can cause problem when trying to scan the same files at the same time. Better safe than sorry.

OK. Seems overkill to me, though.

I have three anti virus systems install on my pc. At first, all three rejected each other. I had to put a couple of firewalls on hold, in order to install all three.

When I restarted my pc, I expected big rejection problems. To my surprise, all three are working with each other.

The only way to tell is by trial and error and see what develops.