I don't think Doc's been Megamorphed, since it was never shown or mentioned and cause his appearance hasn't changed a bit from his Guardian design.
I'm quite sure doc is mega morphed because he would be powerful enough...
The last time Doc was fired, the transformation sound is really the same to the ones when Burpy became Mega Morph.
Tbh I think guardians can't become Megamorphs at all, as Doc's appearence did not change at all if you look at The Goon's Megamorphed form it's different from Doc's Velocity form. Not much but it is.
I don't think that they aren't megamorphed, but are. Basicaly, Guardians are a different kind of megamorph.