Why do you want to slow down the spindle? Most are trying to speed it up for aluminum. Any HSS endmill 2 or 3 flute will work as long as you stay close to the chip load. I have a 4,000 rpm spindle and I wish I could get it up to 20,000. I would be able to move through the material that much quicker. If you are use to working with wood just think of aluminum as a really hard wood. Now if you want to do steel that is when you need to slow the spindle way down and it is pretty much a completely different beast.
Biggest problems Diy'ers face when trying to cut aluminum is the machines rigidity and feeds and speeds. If your machine is shaking all over the bit is to so it wears it out quicker and applys more stress to the bit. If your machine cannot match the feed speed of the spindle it will warm up the bit and instead of burning it will well aluminum in the flutes and pretty much stop cutting until they are removed. Both will cause issues.
You can go with a vfd and spindle the this will allow you to slow down and still maintain torque. You will want to reach the "Chinesse Spindle" Thread in this forum for more information. Eitherway good luck with your adventure
Here is an example of a router cutting aluminum. "http://www.youtube.com/watch?v=MLTAeE6j4_s&feature=related"]YouTube - My home-made CNC router milling some aluminium