The fixed dishes should give a 1pt bonus for every 10pts of skill you have from ability or clothing. So for example if you had 20 decomp you'd get +2 from the dish, 50 decomp +5. I think it caps at +5.
Not home to verify, that's the way it worked last time I looked into it. Haven't done any testing with the portable version.
I think the portable dishes are stacking :S I had +16 to code writing from a crowd of portable dishes two days ago.
Weird, they were not stacking when I went next to the big booster with two smaller ones near it. Thinking about it, it would make sense for them to stack on a realistic point of view, more signal = better ability to transfer information. But from a games point of view, it shouldn't stack because that would take out a lot of the work.
Also, the mini-boosters were overpowering the large boosters, I was only getting +2 on everything, but that should be a known issue now.