There are many ways of adding the sand with water already in the tank but you really should have waited till you have the sand and live rock before you put the water in.
Since it sounds like a new setup, turn off the skimmer. There's nothing to skim yet so your just wasting electricity and putting needless wear and tear on the motor. Same with the HOB filter and the heaters. I'm assuming that the skimmer is HOB or do you have a sump? At any rate, you won't need the skimmer online till at least a month after your tank cycles.
I would just save the water in a Rubber Maid garbage can with a power head to keep it stirred up and aerated till you need it.
You should research what fish you want before starting all this. A tank needs to be built to suite your fish choice. Not the other way around.
Once the sand and rock get there, put the dry rock in as a base. Then add the sand. Then the live rock. Arranging the rock the way you want it making sure the sand and rock stay wet. Last but not least, add the water. Restart your HOB filter and fire up your power heads and heaters. Set your heater to 76-78 and use a digital thermometer to double check your temp. You should only need one heater depending on how large it is. Keep the second one offline for a backup.
After all that you have to wait for the water to clear and the cycling period to begin. There are many ways to kick start the cycle. My favorite is to put a small piece of raw table shrimp in a piece of nylon stocking, tie it closed, toss it in the tank and let it start to rot. Keep testing your water for Ammonia. Once that starts to spike, toss the shrimp and test for Nitrites, same with Nitrates. Keep testing all three every couple of days.
Once your Ammonia reaches Zero you should start to see a decline in your Nitrites. Once your Nitrites reaches zero, you should start to see a decline in your Nitrates. Once your Ammonia and Nitrites drop to zero and your Nitrates drop to at least .25, do a 50% water change and test again in a couple of days. This should all take about 4-6 weeks at least. If the levels are unchanged, then you can add a cleanup crew. Snails and hermit crabs.
Now you can start ordering fish, one by one, so you can quarantine each one of them for a month or longer before you introduce them to your tank.
There are so many things I'm not covering here but you really need to continue your research before you begin all this. It'll save you a lot of headaches, sleepless nights and loss of livestock along the way.
There are no plants in saltwater. Only algae. Some may look like plants but they're algae just the same. Algae are a whole new avenue of research to do.
When you say "Polyps", I'm assuming you mean coral? Coral is a whole different ball game than fish. They require different parameters than fish. Warmer water. Higher salinity. More powerful lighting. Not to mention the tank has to be established before coral will thrive in a closed environment. Some are filter feeders. Some require target feeding. Some have algae in there systems that provide food for the coral. Some need all three. Some need powerful lighting while others don't.
See what I mean about doing your research first? I'm not trying to intimidate you or frighten you off. It's just that none of us here want you to just dive into the deep end without knowing how to swim, have something, or worse yet, everything go wrong and your tank crashes causing the loss of all your live stock and hard work. Then get discouraged and leave the hobby totally fed up and selling everything for pennys on the dollar just to get out of the hobby.
Read everything you can get your hands on, then ask questions here. Then read more and come here and ask more questions. There are people here that have upwards of 30-40 years experience in this hobby. Professors, Chemists, Scientists and so on. We even have a fish medical expert here. Btw, I'm not one of them. Compared to the experience they have, I'm still a newbie. But we all want you to succeed in this hobby and it is an awesome hobby. And don't rely on your LFS (local fish store) for answers. The bottom line is, they're in it to make a buck. More than likely they'll tell you the answers you want to hear, sell you the most expensive piece of equipment when the less expensive one will give you the same results.
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There are many ways of being successful with your tank and get the same results but you need to know what questions to ask to find what suites you best.
Again: Research, research and more research. Remember this, the only stupid question is the one not asked so fire away.