Yes, MTB wheels work fine for cross/low-psi gravel. A Flow is on the wider side, but the EX is narrower than the MK3 at least, so you should be ok. Just go for the wider tire if you have an option. Something like the 40c Kenda Happy Medium would be great. Fast rolling center with some knobs and would only need like 30-40psi.
As Pearl said, MTB wheels generally aren't built for high pressures. I've usually seen like 45-60psi max ratings on some aluminum MTB hoops. That being said, I did put road tires on a MTB wheels once in a pinch and survived at ~80psi. I took it easy over bumps and cracks though in case it blew the rim wall off. 45-60psi is probably more to cover their ass though, kind of like expiration dates on food. Everyone knows you can eat a Twinkie years past the printer date.