What do yo mean by "I am getting a laptop, but want to also have the option of using a Mac Mini with it. It has an HDMI port, so I think I could plug in the Mac Mini".
You can't connect two computer via their HDMI output ports.
A Mac laptop does not support Target Display mode
Use your iMac as a display with Target Display Mode - Apple Support
You can use Screen Sharing over a network to control/use one Mac by another Mac.
http://www.macissues.com/2014/05/07/how-to-set-up-and-use-screen-sharing-in-os-x /
Just what would you like to accomplish?