Yes, it is a 2.5 gallon batch. That's how I brew. I have four three gallon better bottles. I brew in a four gallong pot, and I mash in a five gallon cooler. I don't sparge. Just throw all the water in. If I brew anything big I will have to sparge.
I was going to replace the flaked oats with flaked barley. I have some left over from another recipe, and after a little reading it looks like it might be a similar swap.
I'll let you know how it goes.