Glad to hear you gave up on the Skilter idea....I've known several people that tried everything to get them to work right and it just never happened.
That's cool getting those fish and all. My son was going to Maderia Beach Middle school a couple of years ago and on one of their science outings, he caught a sea slug and brought it home to me. The thing was huge! Pretty cool, but later on I found out that when stressed, this kind could release a toxic ink into the tank so I turned it loose again. The slug looked like a lettuce nudi, but the back "flaps" were not so frilly and the animal had a kind of mottling on it's body. The slug could swim with those flaps too! Very odd to see....
About your sand.....my only concern was contaminates, not silica. I use Quicrete playsand in my 55g and it's silica based too. Aside from the normal diatoms that just about any newly set up tank gets, it really hasn't been a problem.
Stocking rules for inverts......well, there really isn't any. At least, not that I know of. The most important thing to consider with inverts is compatability (this applies to corals too!), lighting needs for corals, and room to grow. Some corals get huge, some have sweeper tentacles. Some shrimp will get along fine with other shrimp, while some will hunt down and kill all the shrimp in the tank. Some crabs will be fine with fish until they get a certain size, same with some brittlestars. Best thing to do is research whatever you catch to see what it's tendancies are.....
Good luck with this, it sounds like alot of fun!!