Is it legal to dim a receptacle outlet? Meaning one combo switch plate with a receptacle and old school circular dimmer knob. An nec code reference would be a big help

The only reference would be found in the NEC 2008 edition Section 404.15:

(E) Dimmer Switches. General-use dimmer switches shall be used only to control permanently installed incandescent luminaires unless listed for the control of other loads and installed accordingly.

To insure operation and intended use, it would be wise to label the receptacle as "Controlled by Dimmer", as if it was used for an appliance, or something other than a light, the appliance will not operate properly, and possibly cause damage.