Please dont think I am lecturing here I am just passing on advice from my experience just dont want you falling into the pit falls I see a lot of people doing
Its a shame I see this sort of attitude form LFS`s, quite frequently from certain chains and "Larger" shops, then we are left to clear the mess up as a specialist marine shop
They are generally are used to selling trops where a neon costs 10p and they shoft 100s of them as they breed like rabbits a lot of marine stock comes from the wild and is expected to live on average 10 years !! I have a customer who has a regal tang that is 15 years old! (In a 300Gallon Tank)
we have refused to sell fish to peopls who want to stick a pair of clowns a regal tang and a moorish idol into a nano, we advise them of the what fish they can keep in the tank I frequently spend upwards of an hour explaining things but sometimes they just ignore our advise, Then they go up the road buy a regal tang from garden centre wipe the tank out then give up the hobby.
Live Rock needs time to adjust to the bioload you are adding to it adding to many fish to the system to soon will cause ammonia spikes which will kill inverts, Fish and corals very very quickly. Nice and slow and the LR adjusts nicely to its load as long as you have good flow open structures and 20 - 40x tank volume.
The next common issue is over stocking the rule of an inch to every 2 gallons is a very very rough guide but is a safe limit as you become more aware of the tank you will understand its limits and if you can push them or not. Once phosphates and Nitrates get out of control they will soon kill off Corals and Inverts although fish are a lot more tolerant. These are very hard to get back under control I can testify to this it took me 6 months to get mine under control on a second hand system
Marines are a lot more sensitive to space than tropicals as most are aggressive there are very very few passive Marine fish this also means you need to add your least aggresive fish first so they have time to carve out there territory and most aggrerssive last this should make life easier. So stocking plans need to be thought out before adding the first fish.
I have some unusual/hard to get fish in my tank and I was pushing my like on a couple of purchases as I had to take them when they where available and 1 fish a week for a couple of weeks, using LR straight from my old tank. There are more fish I want to my tank but with Nitrates at 5 and phosphates at 0.003 I will not push my system further.