I wouldn't want to risk killing off the bio-filter by turning off the sump. That could send you into a mini-cycle. Especially if the sponge isn't as "mature" as the sump.
Personally, I think the screen is the way to go for now.
Tossing the other sponge into your sump is a great idea for prepping the 10g. While you're at it, you might as well toss some aqua clear bio-media in there too. (unless, of course, you're planning to only do mech with the AC.)
You could also try netting them out with a brine shrimp net. What do newly hatched fry eat? BBS, infusoria, hikari first bites? (I really don't know....)