Ya for sure, when I bought the live rock it wasn't in a tank it was just dried out waiting to go into a tank. So the place I bought it from gave me a bag full of water from their established reef tank to help the right bacteria start to grow on my live rock to speed up the process of the mini cycle!
My suggestion is that you don't ask this person for advice anymore...lol. This is some crackpot advice if i've ever heard it. I don't want to put LFS employees down because most of them are hobbyists themselves and I think it's wrong to just say they're out to take people's money, but then i here things like this and I wonder. Either that employee just knows absolutely nothing about the hobby or he's looking to make a fast buck. There are 2 employees in my area i trust completely. This is out of 5 fish stores, and probably 3-8 employees at each. I haven't talked to all of them, but i've only found 2 that will actually talk someone out of buying something if it isn't right for their setup. In my opinion this is the mark of a trustworthy LFS employee. One thing i do is to get a question that i know the answer to, and see what they say. My usual is "can you do a yellow tang in a 55g or 75g?" If they say "yes, absolutely fine in either." Then i don't really ask for their help anymore. I feel this question is in a grey area for a 75g, so if they tell me "eeeehhhhh it can be done, but it's a stretch and likely won't last in the long run," then i'll have a little more faith in their advice.
Not get down on you here, it's natural to assume you can trust someone in a fish store. Anyway, definitely keep an eye on ammonia levels. NitrIte isn't a concern with most SW fish so don't sweat that. I made a similar mistake when i started out and had an ammonia spike, the fish were gasping for air at the top of the tank. I ended up getting a 5g home depot bucket and using an airstone for a few days until i could get ammonia back in check (my tank was cycled, i just added too much ammonia to keep the cycle going the day before and i didn't check the levels before i added the fish). 5g buckets are cheap secondary tanks, but you'll probably need an airpump.
A note on Prime, it only binds ammonia into non-toxic forms for up to 48 hours. So if you do a triple dose of prime, you'll need to do probably a 25% water change every other day and keep dosing to keep the fish safe. If you don't do water changes along with adding it, you'll end up with very high levels of prime after a few times.
Fortunately, it's not an insurmountable task, it's just a pain. It bothers me when fish stores do this because doing water changes every 2 days is a pain, and probably turns people off from the hobby. It just makes things harder than they need to be.
And the advice on waiting 6 months for the anemone is very good and will make your life easier. I started up a 75g in August (after having 75g and 125g cichlid tanks for 2 years, so i was more experienced that the average beginner), and i'm still not planning to add an anemone for a few more months. Make sure you can keep your nitrates at a good level for several months, and you're really confident in your skills. It just takes time and patience. If you add an anemone and then find out your parameters get out of whack, it'll be a massive pain to keep it alive. It sucks, but patience really is a virtue in this hobby. I think this is something most of us have learned the hard (and expensive) way. The way to keep costs down in this hobby is to take your time.
