Getting a Saltwater setup have some questions

You can clean the crushed coral thoroughly with a gravel vac and add the sand, being sure to mix the sand and cc very well. This will cause a massive sandstorm, but the animals will live just fine. The cc will make the sand easier for burrowing fish to make tunnels in. Otherwise, take it out and add sand. Reef inhabitants can deal with a sandstorm, just don't bury anything under several inches of sand or it might die.

It would be better to know what kind of starfish and what kind of hermits before we advise on the cleaning crew. There are carnivorous and herbivorous species of each. Also, what fish are you planning to have or inheriting, as that alters choices too?

125-150 lbs is enough for a lightly stocked tank. If it's heavily stocked, you'll want 200 lbs or more.

Find out if the tank has ever been treated with copper. That's important. Does the tank have corals? If not, do you want corals? What is it using for circulation? There are lots of questions that will help us to give you better answers.

Hmm...

You are probably going to have to move most of the rock and fish to buckets or similar to move things. You might want to handle the sand and PVC while the tank is drained. Mixing in live sand is quite beneficial, but not absolutely necessary. If it is sitting in a sealed bag on your LFS's shelf, it isn't very live. Then add maybe half the water, put the live rock back in and finally add the fish and the rest of the water back in. Make sure you've got some airstones or powerheads or something running whereever you keep the fish as you temporary measure for moving. You don't want them to run low on oxygen in the water. Try to keep the move pretty quick, or change the ambient temperature of the room to similar to the temperature of tank water, or you might need to consider some fans or heaters depending on whether the room is much warmer or cooler than the water. I'd be most concerned about the anenome.

With those animals, you can probably add a wide variety of snails: cerith, astrea, nassarius, and a handful of mexican turbos and nerites. Astrea, turbo, and nerite often clean the glass. Cerith and nassarius often clean substrate. Astrea, turbo, and cerith often clean the rocks. The nassarius prefers meat some meat/detritus while the others eat algae. If you add more hermits, you might want to stick to dwarf blue legs, red legs, and scarlets from least to most peaceful. Lysmata shrimps (skunk cleaner, blood red fire, and peppermint) might be good additions. A serpent star or two might be good, too. With the tangs and angels, you mostly need detritus cleaners and film cleaners, not large growths of algae cleaners. Your royal gramma and anemone are carnivores. The rest of your fish are omnivores and the tangs and angels really like to graze on algaes.

IMO it would depend on if you put the sand on top of the coral or not. I'd take it all out about 1/4 at the time so as not to recycle your tank. The c.c. won't compact and will allow more o2 to get into your sand bed and sort of defeat it's purpose. As to the critters I'd take them all out. It will kind of be like living in a dust storm for a couple of days everytime you add sand or take out coral. That's going to be a big pain though and I'm not sure that some of you critters will be able to take the shock. The anemone being #1 on that list of course it won't like the nitrates from the crushed coral either.
You might want to consider trying a fuge with a dsb give it some time to get established 1 to 2 months and then remove the c.c. from your tank and converting to a dsb you could move all of your critters to the fuge in the interm. Large rubbermaid tubs work pretty well and are cheap. It would also greatly increase your h2o volume. This is just an opinion though so take it for what it's worth.
hth
Chris

Maxilaria,

The point of mixing cc and sand thoroughly would be to keep the bacteria from the cc and pack sand into the crevices and niches of the cc to stop it from producing nitrates. You add maybe half the sand to the top, mix thoroughly and add the rest of the sand. This also gets some aragonite in the bottom to help neutralize some acids that can build up in the lower layers of a DSB. I recommend doing this process while the tank is already torn apart, as it must be to move it.

Kiyana,

However, it might be a very good trick to lay down the sandbed while you've got the tank taken apart and move the cc (or just some of it) to the sump, or vice versa. The live rock should host enough bacteria to colonize the sand, but keeping the crushed coral in the mix can't hurt until the DSB becomes live.