Hi Leslie, glad you like it and welcome to RS !
I'll try to hit your questions one by one
1.The deli shrimp - use a small on in a net bag (to keep the particulate matter down) it will stink and you will need to remove it after a few days. But you will need to add another one of two as your tank cycles to continue to feed your tank during the cycle. you should cycle in < 4 weeks sinceyou are using live sand, rock etc. There is a great thread on RS by a dude named PROW that was invaluable to me through this.
Check it out here.
Newbies, a little something on cycling/breaking in a new reef tank.
2. That skimmer will fit under the lid but you will have to take out the filter pad. if you are using live rock as filtration it will suffice, just put some floss in chamber 2. Also, loose the bioballs in chamber 2 and the spnge in C3 if you are doing a reef tank. They will just increase nitrates.
3. I considered the MJ 900 but so far the stock 600 is adequate for my tank. its almost too much in parts. until something demands it, i am leaving it stock. So long as you dont stack up against the back wal you shoul get adequate flow.
Good Luck!!