30 gallon long - is that 36' x 12" x 16"?
I'm a believe in placing baffles in the sump top maintain a constant water level on the center section for the skimmer.
when the system is running, the water level in the tank will be higher than when it is not, the difference in water will run back into the sump, so you need to make sure you have enough space to accommodate it.
I usually put baffles at 9" in height which will use about 17 gallons of your 30
if your 120G is 48x24 - 2" off water volume (worst case) will go back into the sump - about 10 Gallons putting you at 27 gallons in the sump.
you should be good with that size, though a 40 breeder or a 50G would provide more safety margin.
I like the traditional over-under-over baffle leaving about 9" in the return pump area (about 1.25-1.5" between baffles), I find that this is a nice amount for evaporation with an ATO. this space is about 4 gallons - without an ATO with a 120 you might not be able to make it for a full day without topoff.
depending on skimmer size you might be able to put a 6-8" refugium section to the far side of the sump.
the challenge is fitting a skimmer in the center section with a 12" deep tank
whatever you do, please use glass baffles and aquarium safe silicone!
this setup would look a lot like this (different size tank):
and:
a nice changeup from the traditional sump is to place the return and skimmer on one end, the refugium on the other, and the return section in the middle like this:
