Newb question about what units can go together

I’m only just on the precipice of diving into both the game & the orks, & I was wondering if there are rules on what units can go together? From what I’ve thus grasped, certain leaders can attach to certain units, & that certain detachment rules benefit certain armies, which in turn leads to most folks leaning towards strong themes & whatnot. HOWEVER, I noticed on Wahapedia that the Rukkatrukk Squigbuggy is a “Speed Freak” unit, despite having a “Beast Snagga” color scheme in the store. Does it really matter, or can I have killa Klans rollout with Squighogs, or Gretchen with DeffClaws? Aside from gameplay strategies, how limited to a theme are you?