I've seen a few people do this but it's usually more effective to buy a tinted or mirrored lens. You don't run into fogging issues and don't suffer from peripheral vision limitations.
This true. I used to play with prescription glasses under my mask and my vision is severely limited, not to mention the fogging. I ended up losing a pair of glasses because it got too fogged up so j lay it on the side and somehow it got up and walk away. Anyways great excuse to get contacts instead. So you're best bet would be to buy a smoke lens.
I've never played with sunglasses under my mask, but I've played with normal glasses. Like everyone else is saying, it's gonna fog at some point and they glasses could slip on your nose when you start sweating.
A smoked lens would just be more comfortable in general. I think it's just an issue of preference, if you like wearing the sunglasses under the mask, then go for it. If you prefer buying a lens, then that works too.
I always wear sunglasses with my mask. This eliminates the glare from a reflective lens giving you away, and also is a lot more comfortable when I'm in the staging area, as I don't have to wear my mask to have UV protection, which is necessary up here in Colorado, where sunlight can be very intense. Whatever floats your boat, man, but I like my sunglasses. Polarized or smoke lenses never really worked well for me, but they may for you!