Ron,
I've done a lot of work with solid surface material and other acrylics, on the mill, lathe and with routers. My preference is always for a spiral fluted bit, solid carbide if preferable. The spiral flutes give a shearing cut to the material and provide somewhat of a bearing surface on the leading edge of the cut because except for a very small amount, depending on the angle of the helix of the flutes and the depth of the cut, the leading edge of the bit is always cutting. With a straight fluted bit, 2 flutes, the bit cuts the full depth of the pass at once, then doesn't, then does, then doesn't...etc as the bit rotates. Also the spiral geometry of the flutes will lift the chips out of the cut as you cut (assuming a right hand cut, right hand spiral) which will help keep your cutter sharp. The essayist way to dull a cutter is to let it rotate in chips. Acrylic is pretty abrasive. It will also keep you cutter cooler, which is important in solid surface material because heat will cause stress cracks...especially in the bottom corners of the dado if the bit is dead sharp.
Solid carbide is preferable to brazed carbide on steel because the carbide is much more ridged than steel, and you are taking a pretty hefty cut if you do this in one pass. I would want the rigidity of carbide. I would also be very good if you could slightly radius the sharp corners of the bit where the edge meets the end. This is difficult to do if your not trained because you still have to maintain the back rake on the edge as you radius with a diamond hone. I'm talking about a .005 to .010" radius max. If properly radius-ed with the proper cutting clearance, the bit will cut much better, longer, and the dado will be much stronger. A good sharpening service or local tool and die maker could do this for you in about 5 minutes with a diamond hone....but....you have to know what you are doing or you will ruin the bit.
About the size of the cutter- Festool makes excellent equipment and accessories and their quality is top notch, but EVERY router and collet system has some run-out inherently built in. if you were to use a 12.7 mm bit and take this dado in one pass, I would be pretty certain that you would get a slot 13mm or more due to the collet run-out ( there WILL BE some). You can force collet run-out if you need to by slipping a small strip of cigarette paper between one side of the bit and collet- an old toolmakers trick for cutting slightly oversize with an end mill.
If the slot (dado) really needs to be 13 mm as close as possible, you wouldn't want to cut it with a 13mm bit. You would go undersize and rough out the slot to depth and the take a light clean up pass on one wall- maybe .010" and then sneak up on the dimension on the other wall. I don't know how exact the width needs to be. But you can set the router movement with the fine adjustment on you guide stop, and a dial indicator clamped to the material.
I hope this helps you. It is extremely complicated to cut a dado that long, that deep and hold tight tolerances. All the details must be perfect. I don't know what your tolerance really are or need to be. Good Luck
Jay
