I think more info on your setup would be helpful. Are you using belt reduction in conjuction with rack and pinion? Leadscrews or ballscrews?
Yes there is a tradeoff between resolution and accuracy with steppers. You may have to use a lower microstep setting on your drive. Or you may need a faster computer, or even an external pulse generator like a SmoothStepper. You may have too small a power supply.
It may also be the timing pulleys used for the belt reduction; you could easily swap one pulley to get a different gear ratio. This may require swapping the belts as well.
As for aluminum your feedrate is ultimately determined by the spindle speed. There is no "correct" speed, though aluminum machines nicely at faster speeds, provided your machine can handle it.
I try to achieve a .0027" chipload when cutting aluminum with a 1/4" endmill. This is a tad lower than the recommended chipload (.003"-.006"). At about 12krpm that comes to about 32ipm per flute. In fact I (and a few others here) use single edge spiral-O-flute bits in order to maintain a decent chipload. At 19ipm you'd have to be at about 7000rpm with a single-edge spiral-O, and about 3500 with a 2-flute bit. Once you know what speed to use, it's a matter of figuring out how much depth per pass your machine can handle; I try to acheve 1/2 the bit diameter for pocketing and 1/4 bit diameter for profiling, due to my machine ridgidity (or lack of).
The above is only a guideline, you may have to tweak a little to get it just right. Basically if the aluminum chips are not hot to touch, you're not doing it right. The basic formula is
feedrate = chipload * number of flutes * spindle speed
and you can derive any one by knowing the other three.