New to the hobby. Few questions

[Coral Hoarder]

Hey, welcome to the hobby

So you can definitely mix rock I wouldn't thought tends to look off instead I would get a few pieces of established rock to put in the back compartment .
Definitely buy new sand sand is cheep.why use someone's gross old problem sand. A few extra bucks goes along way. Definitely start by adding a few snails and hermit crabs when you start getting an algea bloom i would recomend doing a fish less cycle as its kinda hard on the fish.
The best way to clean your tank would be to go to Walmart and buy 3 gallons of vinegar will cost like 6 bucks an mix it with enough water to run the tank till its clean