I'm guessing you've chosen acrylic over MDF as you're intending to use coolant? If not I'd go for MDF as you can screw things into it much more easily and it's cheap.

The 6mm since flute carbide cutters I keep mentioning work well on polycarbonate / nylon so I should think they'll be effective with acrylic. For levelling the bed having clearance for the chips isn't a problem so you may be better off with 2 flutes ... then again the former is much more useful in general. Shame you can't use 1/4" (?) router cutters, as a big one of those would be much faster and probably sufficient.

The little router at school used a white plastic bed, not acrylic...not sure what it was precisely but it was easier to cut and we did level it with 4mm 2fl cutter, which took forever, as that's the biggest the router accepted.