if you mount 26 mm tire on 24 mm rim thetire will have very pronounced dome and will more than likely have a very small contact patch. another option is mounting 24 mm tire on a 26 mm rim, that will give you avery lo profile since you are spreading the sidewalls out further howver you will loose sidewall support during hard cornering. ideally you should match the tire to the appropiate rim, 24 to 24, 26 to 26.