The anemone-thing does look like a sponge to me, agreed with the above.
Your anthill with antennae looks like a spaghetti worm. Great detritivores. They may annoy any corals right next to them by touching the corals, but are generally considered to be harmless.
The crab looks like maybe an emerald crab? At that size, no matter what the species is, it can't do any harm. Definitely find out what it is in case it gets big, but no need to worry about him.
The bristleworms are just that, yep. They freak people out, but they're very useful.
Nice find on the brittle stars! How big are they? You could always trap some out and give them away. I'd definitely start feeding, though, you don't want all those good scavengers to starve.
I'd be interested to see a few pics, in a week or so, of the difference in algae growth between the dry and live rock. I mean, I'm pretty sure I know what it's going to look like (pest algae running wild over all the free real estate, maybe the start of non-pest algae catching up, and the live rock probably looking mostly unchanged), but it'd be cool to see it illustrated in the same tank.
A mix of dry and live rock is great. Dry for all the cool shapes and the ease of scaping, live for the critters. It's what I'm going with when I can eventually set up a bigger tank. I went with all live for my current tank, but it's a pico, so there wasn't really space for both!