Sidis are some of the narrowest shoes out there, they also have a pretty basic stock insole. I replaced my Sidi insole with Aline orthos. My LBS actually does shoe fits, maybe you can find one that offers this?

As for the other question about mountain vs road.... A stiff shoe will help you get the most of out your effort. I also find it more comfortable for me. My mountain shoes (Specialized Riata) have a fair amount of flexing, I wasn't too happy using them on my old commuter especially if I got out of the saddle. That being said some of the mountain shoes are made with a stiffer sole so it may be a different feel. Many people use mountain shoes on their road bikes, it just isn't for me. But then again you may hate trying to walk in road shoes or putting your foot down to stop, it is a different feeling from mountain shoes.