In my apps I also have the habit to add a "markup-style" column to my db-tables containing the text-content. This allows me to change the markup later on. You never know what might happen in the future. If you have this field (in my case usually an integer) available, then you can decide to either convert the old formats to the new one, or (IMO the better solution) handle them conditionally.
Yes. Especially for mobile browsers. I created a site recently using the 960 grid system. This also uses (as the name suggests) a fixed width of 960px. Using this on a WVGA (800x480) mobile device is not really that usable.