Yeah sorry if I wasn’t clear. I know you don’t currently have a wrasse, but didn’t you have a thread earlier today asking for fish recommendations? In that thread the wrasses i mentioned were recommended, and if you get a wrasse now, that will effect your copepod population.
I do know what a copepod hotel is. I haven’t seen anybody write about their experience with copepod hotels but I haven’t looked either. I would want to know if fish really can’t get into the hotel.
Like everybody is saying, I know you can find some people who will say you can keep a mandarin in a 20 gallon tank. You’ll find a lot more, including the people who breed captive bred mandarins, saying you need a much bigger tank *and* a refugium *and* an established population of copepods to be successful long term (more than a year).
Here is an article from AlgaeBarn (and they sell captive bred mandarins).
Learn how to keep your Mandarin happy and healthy using live foods.
www.algaebarn.com
As the poster above said, and your LFS said, as long as the mandarin gets enough to eat, they will be ok. The other side of that coin is that to make sure they can get enough to eat, you likely need more live rock than can physically fit into a 20 gallon tank and/or a refugium for breeding copepods and/or the ability to regularly add copepods to the tank. I would look up the price of bottles of copepods to see if you’re ok paying this every few weeks.
The good thing is that you seem really invested in getting a mandarin and there are things you can do now to get yourself ready to take care of said mandarin. Set up a refugium. Set up a pot hotel and see if it works. Learn how to culture copepods and phyto (to feed the pods). Think about whether you can get a bigger tank and more live rock. You can do this!

I just think you’re going to need to take some active steps before you get your mandarin.