can a iMac mini be connected to an older iMac retina 5k 27" ?

Review the document: Use your iMac as a display with target display mode - Apple Support

Target display mode is a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.

Two things I'll add:

1) There is no "iMac mini", there's only various Mini models.

2) You do not provide the year information for the Mini so I provided the information covering the models that are supported. For all we know, you are asking about an older Mini. The constraint is the macOS version that they will run, and newer models cannot run the older macOS versions required for Target Display Mode use. You will have to determine if your Mini is one of those.

No, the new Mac mini's require an external Display.

see > Connect a display to Mac mini - Apple Support