Don't worry - no harm is possible.*
Guitarists use effects loops on their amps all the time, even for pedals, because there is a general preference for relying on preamp distortion as opposed to pedal distortion. With bassists, it's a different story.
However, many pedals don't "like" the output of an amp's effects loop, and they'll distort or even decrease your preamp output. But again, there's no harm to your pedals - just unpleasant distortion in many (but not all!) cases. If it doesn't sound right to you, just take it out of the loop. Simple as that.
In short... it narrows down to what the pedal is designed to handle. By convention, the vast majority of rack effects are designed for effects loops and line level signals. Pedals... not so much.
The pedals that usually work best in an effects loop are standalone ambient effects like modulation (chorus, flanger, etc.), delay, tremolo, etc. - but not all such pedals will work well. It's all a matter of your setup.
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There are a few ways to make stuff go horribly wrong... but if you figure those out, you'll destroy your amp without even using the effects loop. 
