You need to make sure you put !important in all your css declarations so that it overrides Marketo's defaults (see my example). Also note .mktoButtonRow is the button I used, please change to the class of your button in case it's different. Also .mktoForm form width is dependant on your page width, the 1000px I used is an example and again overrides Marketo's default.
It's possble! a field set has a distinctive class in css. You could create custom css to float a field set left for example. I've done many forms this way since Forms2.0.