Can an 2019 iMac be used as a display for a new Mac mini

No sorry, Target Display Mode does not work with any late model Mac's.

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

To put it simply,

1. the new Mac mini does support using any iMac as a display.
2. the 2019 iMac is to new and does not support being used as a display.

It just occurred to me I have an 2008 iMac.... could that be used as a display?

Please look as the Apple Support article that den.thed has linked. It clearly shows the very limited sub-set of iMacs that could act as a monitor for another Mac, and what other Macs could use a TDM-supported iMac as a monitor.

TDM is dead tech. Apple started its deprecation in Late 2014 and threw the last shovel of dirt in its face in early 2020.

No sorry. If you get a new Mac Mini, then you will also need an external display to go with it.

see > Connect a display to Mac mini - Apple Support