What Jay said is very accurate, I have a small return section with not ATO in a 55g tank and I have to top off the return section every other day, and it is larger then yours (still a 10g sump.
Also If you have the room outside of your tank for the HOB skimmer then could you not just use a slightly larger tank and place a skimmer inside the sump? What size tank are you putting this under? If it is a 40B or a 55g, I would use a 20g long as a sump instead (its what im switching my 55g sump to).
As far as baffles before your pump, if you use a HOB skimmer (or any skimmer) in the first section, with a bubble trap following, you will not have much problem with bubbles (as long as you increase the return a little. If you use an ATO and always keep your return section waterline a few inches above the pump it should not pull bubbles. If the water level gets lower then you will need a few inches between the baffle and the pump to prevent the bubbles. I run no baffles on the return section of my 55g's sump and have no micro bubble problems, I removed the baffles just simply to give more room for water as a small return section was frustrating.