to your original question, "how much compressor can change clean tone"? A lot or a little. My experience was the "a lot" version was the Boss CS3 (I think it was called). Got really squishy with that tone, it was almost like an effect. Didn't like it for traditional compression needs and it seemed to change the output and overall character of my guitar. The other side of the planet is a Barber Tone Press. It uses a parallel compression where it compresses the signal, but with virtually no altering of the original tone, and that's what I was looking for. For clean work, an uncompressed signal to me just sounds a little harsh on some strings, and this really smooths it out so every string rings out about the same. It's subtle, but I haven't been able to take it off my pedal board so I'm officially addicted to that subtle improvement. And I like the bit of sustain that comes with it for solo work.
Everyone raves about Keeley too, but in my limited experience it did a little more tone alterning than the Barber (although sounded good), and I didn't like the extra knobs. Simple is good.