I'm looking at buying used frame that takes Megaexo BB. Is that compatible with a standard Shimano 5700 bottom bracket? If not, would Megaexo bb take 5700 cranks or is this like BB30 where you need specific crankset?

Since it has a Megaexo, that frame's bottom bracket shell almost certainly threaded, most likely English, and any compatible bottom bracket can be installed. The Megaexo bottom bracket cups can be easily replaced with Shimano 5700 threaded bearings.

The Megaexo also accepts the same nominal 24 mm spindle that Shimano uses but the fit between the FSA and Shimano spindles and bearings is not always good and it would be better to use the Shimano bearings.

Maybe I'm wrong here, but is there such a thing as a MegaExo frame? The MegaExo bottom brackets that I've encountered have come in two versions: English (68mm) and Italian (70mm) threaded. Shimano stuff like the 5700 BB will work on an English or Italian threaded frame no problem.

As for mixing Shimano cranks and MegaExo BBs, there can be compatibility issues, so I wouldn't recommend it, especially given how cheap it is to get the right BB.

Thanks , this is the info I was looking for. The frame doesn't come with the BB but the bike originally had Megexo. So it sounds like the BB I have will fit if it's standard threaded

I might be reading your statement wrong, but I think you are incorrect. If you have a threaded bottom bracket - English or Italian, you must get a bottom bracket with the matching threading. There are no bottom bracket cups I know of that will work with both. It is a geometric impossibility.

What I mean is that Shimano BBs come in English and Italian versions, not that the same BB fits both frames!