I thought the lowest was 500.
In any case, there is less hp available at the spindle at 10,000 than there is at 6,700 due to friction in the spindle bearings. additionally the lower belt contact angle means the belt needs to be tighter.
you could run a half inch diameter endmill at 10,000 rpm in aluminum, but your machine doesn't have enough hp to make anything but dust at that rpm, and you also have to find an rpm that produces a good surface finish due to the incredible flexibility of that machine.
if you have access to a welder, you could build a larger two piece weldment, a copy of the frame the machine bolts to now.. and machine the mounting surfaces on your machine. I would go with a minimum of .25" thick 4" square steel tubing.
rpm will need to decrease with depth of engagement due to hp limitations as well. 10,000 rpm on a .25" end mill might work on that machine at .125" deep in aluminum, but if you're cutting .75" deep you'll need to drop the rpm and feed rate down.
Tooling Pro
your machine might have enough hp at the spindle to cut a .25" wide .25" deep slot in aluminum at 8 inches per minute. try doing this at 4200 or 2600 rpm spindle speed.