Your iMac will be able to run Yosemite. When we initially installed Yosemite a year ago we had a few odd problems with Photoshop (that might also have affected InDesign). But these seem to be ok now, I guess updates have been issued. It shouldn't slow things down, but updating an OS is always a good time to have a bit of a clear out. Make sure you back up first. You won't have to reinstall Creative Suite.
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