panarchist

3rpm is considered by some sources to be the practical limit - it's a bit more generous than 2rpm. USAF tests in the 1950s/60s showed that more than 80% of test subjects could tolerate 3rpm just fine, and a significant number could handle up to 10rpm. The big issues at those rotation rates come when you try to turn your head quickly - apparently not fun.

Putting a decoupler atop a bigger decoupler in the same stage is not a recommended configuration. You *will* void your warranty. Fortunately for the brave Kerbal crew, the lander survived the incident intact, and they actually *did* go into space today. But it was a very near thing. Unfortunately, the 6 nuclear engines fell back to Kerbin, and the crew did *not* go to Duna as planned.

Jarin - I've been using this ship here: http://vanillaspaceagency.com/pathfinder-mk-8-nuclear/ (all stock parts) Multiple decouplers will bounce off each other and cause the engines to go flying. I was able to successfully decouple them manually by right-clicking and doing it individually - the space bar does exactly as you describe.