re: increasing feedrate when decreasing stepover. When your stepover gets very small (less than 10% the tool diameter or so,) chip thinning starts to really come into play. I'm not going to explain it here, but John @ NYC CNC does a pretty good job of it
(he's got a lot of decently useful videos and I find them to be fairly entertaining as well.)
For a machine as light as yours, you want to be using the minimum chip load a tool will tolerate as well as narrow stepovers and shallow DOC. Most good 1/4" carbide can tolerate down to .001" chip load, really probably half that. It's probably not a bad starting point at a narrow stepover though. I'd try a 2-flute, short as you can find 1/4" and 1/8" carbide flat end mills. Chuck them up in the collet until the flutes are almost in the collet. Run 12k RPM for the 1/4", 24k for the 1/8", .03 stepover and depth for the 1/4" and .015 stepover, .03 depth for the 1/8" and around 24 IPM for both.
Try these on straight cuts on the edge of a work piece. Once you have cut a little, start adjusting parameters, one at a time and take notes. Don't be afraid to increase parameters, sometimes things go better a bit harder.
I'd also try conventional milling as well as climb milling. I'd guess climb will work better for you, as it's a bit more forgiving of lacking rigidity, conventional is more forgiving of backlash/slop.