75g build

My 16 year old son and I are setting up our first reef tank. Planning on adding water this weekend. Here's what we got and what we've done so far.

75 gallon tank with 20 gallon custom made sump.

• Lived on the BRS (and others) YouTube channel for about 2 months learning
• Custom made the sump 2 weeks ago (I'll post pics when I figure out how) lol
• Drilled 3 holes for 1" Bulkheads and added overflow box in the tank this week
• Plumbed the tank 2 nights ago (including about 4 separate trips to Lowe's)
• Ran dedicated 20A electric circuit with to the tank
• After the first RO/DI filter came damaged, the replacement arrived yesterday and I promptly started making water.
• Plan is to add sand, about 120 lbs. dry rocks (plus 1 live rock I received from another reefer) and water later today

Couple of questions I have for the group:

1. When should I get my refugium up and running?
   
2. I'm planning on using Chaeto as well as dry rock in the fuge. Should I use sand as well? How much rock should I use?
   
3. I'm planning on adding a protein skimmer and carbon reactor in about a month or so, after the tank has cycled for a bit. Besides a foam filter to catch particulants, any other sort of filtration I should add from the get go?

I'm open to any and all suggestions because this is our first go round with this.