I've not machined that particular plastic, so I cannot speak for its properties when machining.
However Ive machined a good bit of delrin and other plastics, and I would generally use an uncoated endmill because you want it to be as sharp as possible. Also, you are likely going to have to support the part fully, any chatter will come from the plastic itself bending, not the tool, especially not a 3/8 end mill.
Ive had a job where I had to mill out an octagon shape in aluminum over a 3" length, and it had to be done in portions because of sides being unsupported in the guide. We machined the first hundred thou or so into the octagon and then came in and supported the part with the sub because chatter and flex off the aluminum was bad enough.. 3/8 plastic is basically a noodle and is going to bend out of the way with ease and cause you all kinds of nightmares if you arent planning to support it with your pickoff.
I would personally do your flats in portions, moving the tool in x or y (depending on your machine) rather than to feed in Z.
Feeds and speeds, well, I generally haul ass in these types of material, you shouldnt have to worry about melting with a steady oil flow, but I would say the amount your part is going to flex will dictate just how fast you can cut it more than anything.
what exactly do you mean by from the bushing face, I initially assumed you meant how far the tool sticks out but from reading your post over you mean to say that each tool would have different center positions? Im confused with this part to be honest heh.. I can guess youre er11 tools are closer to the guide than your er16s, which seems odd as ive never come across a machine like this, what machine is it?