How do you all handle the gap under baseboard when the floor isn't in yet?

What the flooring guy told you is the normal way to do it. You set the baseboard so the bottom edge sits at or just above the height of the finished floor, then the gap that's left gets covered with shoe molding or a small base shoe piece tacked to the baseboard, not the floor. That way the trim floats independent of the floor and the shoe hides any expansion gap the flooring needs along the wall. For 3/4 solid red oak, that means your 1x8 should be floating roughly that much plus a hair above the subfloor right now. If it looks like a lot of open space, measure it. If the gap is close to the thickness of your flooring plus a little slack, you're fine. If it's noticeably more than that, you did set it high and you've got a couple options: pull it down and re-nail it lower, or rip the bottom edge down on a table saw to drop the profile closer to the floor. One thing to check before you commit either way: ask the installer exactly what the finished floor height will be. If he's putting down underlayment or building up with anything under that oak, the floor will land higher than the bare board thickness, and a base that looks too high over bare subfloor might be nearly perfect once the floor goes in. I've seen guys pull trim twice because they guessed at it instead of getting a number.

Also hold off on filling nail holes and doing final touch-up untill the shoe is on. You'll pick up a few more nail holes when it goes in, and the shoe needs caulk along the top edge anyway, so doing all your finish work in one pass at the end saves you a bunch of fiddly little fixes.

One trick that saved me a re-do on my last room was getting a cut-off of the actual flooring and its underlayment from the installer and setting it on the subfloor next to each base run, because guessing from a spec sheet and seeing the real stack height are two diffrent things, especally if there's a pad or glue build involved. I'd also run a pencil line snapped off that scrap rather than measuring up from the subfloor in a bunch of spots, since old floors are never flat and your boards will follow the dips if you nailed them tight. Mine sits about a flooring-thickness proud in the low spots and it still reads fine once the shoe went on.

I'd push back on the table saw suggestion in the first reply. Ripping the bottom off a profiled board doesn't just drop it, it slices right through the cove and any detail near the base, and you end up with a raw end grain edge staring at you along the whole wall. I tried that shortcut on a bathroom job years ago and had to buy new stock anyway. If the gap is truly too big, pulling the boards and cutting fresh pieces is less wasted time than trying to doctor the ones allready nailed up. Better to nail a scrap of the shoe molding up there temporarily and see what's actually left showing before touching anything.

One thing to keep in mind is shoe molding only covers so much. Standard base shoe is about 5/8 tall, so if your gap ends up being much bigger than that once the floor lands, no amount of shoe is going to hide it. On my last job I set the base a full inch off the subfloor and panicked until I laid a piece of the actual flooring down, turned out to be near perfect. Also cope your inside corners on the shoe if you're using oak, it's hard as heck to miter tight.