My mill will onlyu handle up to 3/8"end mills and I run 1/8" up to 3/8" all at the same speed only because my mill tops out a3,000RPM. My mill will go do .125 or 1/8" passes at the mills slowest setting, which I have no clue what feed rate it is as it just has 1-6 for speed options. Guess I should time it one of these days. I have a pretty strong motor though. It says it has more tourque than a 1/2HP AC motor and I belive it. It is actually way to much motor for the mill it is mounted on.
You will just have to experiemnt a bit to find the mills optimal speeds per material and end mill. morphlin1 just posted some good info in another thread.