I would say it depends on the type of whiskey and what you are looking to get from the maturation. I am aging gin,single malt, and a light whiskey in used 15g anywhere from 110 to 130 proof. I honestly think 120 is still sufficient for 2nd maturation in a barrel this small for most things especially if you can go 18-24 months.
I would also consider what proof you want to bottle this spirit at. If you enter at 130 into a small barrel (with large angels share) and get a nice abv bump, a lower bottling proof may require a ton of dilution which could be a detriment to the final product. In Scotland they can enter at 130 (in used cooperage) knowing they are coming down 1-2 proof per year.