No rules and don't listen to these old farts. Lol
- Look what the modern day 'compact frame' evolution has done.
- The 'professional' to OlYmPiC fitters and coaches all have completely changed from what was customary from the 1980's and prior.
- Components are available for every conceivable means in making a small frame work for a larger stature. Crank lengths to bring you down from a longer seat post to giraffe necked stems, bar configurations and width purposes.
Pro and cons, surely enough reads and arguments to just say screw it and go try at.
PS. I've seen many riders even touring with smaller frames, and they like the extra clearance, plus room for a top bag, etc..
Many a pro riders downsized as well. Tri / tt folks as well. Smaller triangle stiffness, shrink the body up a bit to break the primary aero drag, etc.. The UCI is a stickler on much of those issues because its THE key ingredient of being fast, figuring it out for a competitive edge.