Hi, I just bought a new Imac with Catalina, is it possible reinstall Mojave

It depends on the year it was made; if it was manufactured and loaded with Mojave originally then you should be able to. If it was released after Catalina was introduced, then no - you cannot. Macs cannot boot from any OS newer than what they came with.

I recently bought a refurb 2017 iMac from Apple and it had Mojave loaded. When I picked it up, I had them erase and reinstall Sierra as that is what this iMac originally came with. That is why I bought that one because I can run some older movie and video software that will not work in 10.15 or 10.16.

No, you can't install an OS older than that installed at time of purchase.