I recently heard somebody say this: "You can think of mandolin like an upside-down guitar. "
What? Who plays guitar upside-down? I know they are referring to the tuning (guitar tuning from low to high starts out E A D G, mandolin is G D A E). This idea looks good on paper, but honestly, falls pretty short in practice.
If you are a guitar player picking up a mandolin as a second instrument, the best advice is to learn it as a new instrument. Learn the chords on mandolin without trying to think of them through the guitar. Learn the scales on mandolin, without comparing them too much guitar scales.
There are plenty of similarities. Fingering notes and picking strings are basically the same on either instrument.
When it comes to chords and scales, though, just learn the new ones on mandolin.