Been doing 1 gal batches and about to step up to 5g. But had been pondering the 2.5 & 3 gal kegs for recipe creation. Any probs with under filling a keg?
No. Since you are filling the head space with CO2, there is no problem at all. In the past, I have bottled half a 5 gallon batch and kegged the remaining 2.5 gallons in a 5 gallon corny. No issues at all.