Flatland bikes are not good for street riding, generally. As Sloth said flat bikes are usually very short (but he's wrong about rear end length, most are around 13.25-14.75), you'll find it hard to get enough leverage. But also, flat bikes don't need to take nearly as much punishment as street bikes and as such aren't nearly as strong. So if you try riding street with something like a Quamen or an Ares, don't be surprised if it breaks, even the longer ones.

If you want to ride a bit of street but mostly flat, I'd suggest either a Volume Mid or a Standard Tao, they're both really strong but have flat-oriented geometery. If you'll be riding street more, just go for any decent street frame with a top tube of around 20 inches, maybe a bit less. A short back end is a good asset for either though.