Can you use this pad with a quiet flow filter? I have a 10gal tank with a 10-20gal filter.

It's designed to fit exactly in a Fluval C4....not sure it will fit your need..

Can I use c2 nodes in a c4 filter

Sorry, I haven't a c4filter, I have only a c2 and a c3 filter. But it's very useful for them.

how often do you change the carbon filters

I change them every 2-3 weeks, but I have very large African Cichlids and they can be messy. After they absorb to their capacity they are no good anyway, so you need to change them, but they do a very nice job keeping the water clear. You should also check out the C4 Zeo-Carb 3-pack. I switched to those and they offer the carbon plus ammonia absorbtion which has worked great for my tanks. They are a little bit more expensive than the plain C4 Carbon 3-pack, but still cheap overall.