To all intents an purposes as far as WSC is concerned it is off because it isn't active and that is what it is monitoring. It is safe to pause standard shield to do a scan with another scanner (obviously being on-line at the time isn't advised), this is to avoid conflict between two active scanners, potentially doubling the files scanned and possible false detection of the other scanners signatures. I always pause standard shield before another scan and ignore WSC's warning.
It is safe to have it on any setting you choose, life is a compromise between performance and protection. I can't say what your requirements are so you should decide, if the High sensitivity is scanning lots of files and potentially slowing your system, allied to that what data do you have that if you lost it would cause lots of grief. On that note, ensure that you back-up that data so it isn't lost, have a recovery plan as to how you would recover from any catastrophe be that virus, hard disk failure, etc.
