Sorry - 20 lbs. may be too light.
D'Addario Mandocello Phosphor Bronze, .022 - .074, EJ78 run about 30lbs. per string.
NOTE DESCRIPTION DIAMETER TENSION LBS
1st A Phosphor Bronze Wound 0.022 31.1
2nd D Phosphor Bronze Wound 0.034 32.5
3rd G Phosphor Bronze Wound 0.048 29.1
4th C Phosphor Bronze Wound 0.074 29.7
With your short scale an 80W C may be needed just to keep you from being floppy.
I'm just plugging numbers into a string tension calculator to try and help. No experience with a mandocello.
Good luck!
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