Can I use my Imac (2009-2010) as an external monitor for my (2020) new Imac?

It will work if it's a 27" iMac, 2009 or 2010. Connect it to one of the Mac mini's USB-C ports with:

mophie USB-C Cable with Mini DisplayPort Connector - Apple

Use your iMac as a display with target display mode - Apple Support

( The "Mac from 2019 or earlier" is for Thunderbolt connection, and doesn't apply for a Mini DisplayPort 2009 or 2010 iMac.)

See this topic: Tested, iMac 2020 STILL supports target display mode with older iMacs

It only works if the older iMac has a Display Port. Using a Thunderbolt port does not work.

Your best bet is to simply buy an external display, depending on your needs they start at about $130. This is a much more trouble free solution and doesn't require expensive cabling.