>I think it was something like 1/16 of an inch on each side.
That's correct.
> My question is, since I still have a lot of assembled frames which haven't been trimmed down, is it necessary to trim them down for small cell use?
It's not necessary. Although, if they are not nailed from the side it's easy enough. As with many things with bees you just try to stack the deck to get what you want. They tend to build smaller cells with the tighter spacing. Left to themselves they tend to build the tighter spacing too. So it's an advantage, but not a necessity.
> Will the bees muck up their foundation if I allow them to draw on these frames?
Not necessarily.
> The reason I'm asking is because it's a pain in the butt to trim these assembled frames down. A manual plane doesn't work too well, and I don't have an electric one (if that would even work, I'm not sure). I've been trimming the unassembled end bars with a table saw, which worked well.
A table saw is much easier to use on assembled frames than unassmebled frames. You have a wonderful handle on the assembled ones to hold the frame while you run the end bar through. I never cut them down before I assemble them.