You don't say what kind of aluminum (or aluminium) you're using, but it makes a big difference. Some are soft and gummy and will machine poorly no matter what you do, and stick fiercely to your cutters. Go with a known machinable alloy that's hardened - harder alloys are actually easier to cut. 6061 T6 is popular in the US, but you'll probably have to check around wherever you are for an equivalent.
To make slots, it's best to rough them with a cutter that's smaller than the full width of the intended slot, taking repetitive shallow cuts down the middle, and then finish with a climb pass to clean up the edges and achieve the full width.
I've never used specific roughing cutters for aluminum, only for steel. Usually a 2-flute cutter is good, but since your spindle is so fast and doesn't seem to like slowing down, you might try a single-flute endmill: Precision Cut Single Flute CNC End Mill, Premium Carbide,Cutters