I've done E-6 in C-41 and C-41 in E-6.
When you do C-41 in E-6 you will basically get a transparency with a nice orangey base. This CAN work, but you have to plan on that base. It will be there.
Now, E-6 in C-41 is a bit different. Since you get a negative with NO base color, this changes things a bit. You get some AMAZING color saturation. You also get WHITE-whites and BLACK-blacks. All your highlights will blow out, and all your shadows will be as dense as fruitcake.
As to exposure, with me, I just shoot Ektachrome VS100 (or whatever it is now) at 100 and get it developed at 100.
Then again I WANT the look you get from that setup. I like the density and saturation.
I also have the prints done to spec. I tell them to "print from skin", which is to set the minilab to make sure that skin-tones are balanced and to let the rest fall where it may. It can be AMAZING in a portrait. You get some neat effects.
You also have to keep in mind there is a bit of voodoo here. If you want something consistent and safe, this is not it! You could, I am sure, get repeatable results, but that is no fun! This is something to be played with!