Use a bit designed for cutting acrylic if you can, single flute works best for me with the speeds & feeds my machine is capable of, when it comes to setting speed & feed this is something best worked out by experimenting as they differ depending on machine, cutter used etc. Cutter size as with any job is really dependant on what size parts you are cutting & level of detail required. I have bits ranging from 1mm to 6mm for cutting acrylic & this is the only thing they are used for
Good dust extraction is very important or you will end up re cutting chips leading to heat build up & poor finish. My dust extraction isn't actually great so I use an air hose to remove chips.
Hold down with thin plastics can be a problem as they tend to bow when clamped so if you don't have a vac table double sided tape or something similar is a good option.