A lot of the answers to your questions depend on your riding conditions. Distance, terrain, loads carried, time constraints, availability of showers or changing facilities, etc, etc, etc. I have been commuting for years and spent a lot of time reading about commuting and although there is a lot of great advice, some of it is silly in the face of what the given commute is. For instance, I see a lot of people asking about commuting who have sub 5 mile round trip commutes across fairly flat ground. In this case, I say ride whatever, however. You could stick a pair of wheels on a granite boulder and ride that. It just doesnt much matter. On the other hand is the person who has a 30 mile round trip that is half gravel trail and half serious urban street biking while carrying a 30 pound load. This requires some consideration and planning.

As someone who has been on the farther end of that scale for some years, I can offer my thoughts, but you don't need them if your commute is two miles down the road.

Tell us about your commuting conditions and needs and you will get better advice.