lighting manufacturers who make flourescents have designed their lighting units to fit onto either a 750 (5/8") spiggot or into a grip head in the case of Kino's lollipop mount. If you're jerry rigging your own you'll have to figure out some way to fit it. A lot of people use a 750 baseplate..
Fluorescents are used as a soft source...they can possibly spill but the light is so soft that generally it falls off pretty quickly before you start seeing stray light everywhere. Using a grid/eggcrate can help direct the light a little more but you won't get the kind of control you can with a fresnel...