Just return them! If they can't tell you the right info, then they should take back their issues. And the Mono will prefer brackish water too!
It is hard when there is such a vast amount of interesting stuff offered at a good lfs
I had to stand tank side and keep looking stuff up on my phone when I recently went in to a nice lfs
, which offered a much larger and different variety of fish than I had seen at other more local fs.
The owner may know almost everything but his employees sure don't. They employee said yes all of the varieties of this fish get along fine in a community tank. Which I knew was true that all the ones I had read about were good for a friendly/ peaceful community. So I was convinced the guy was telling me the facts.
So only because I had already nearly spent my fish fun fund, I decided to go home and learn a little more before jumping into a new kind of fish I had never actually kept before.
I had almost purchased a quad of fish that didn't get along with others in a community tank, even though I knew something about the type of fish, the particular species he had wasn't friendly and would fight other fish which might come near its territory, so not a good community partner. I only found that out after going home and I spend a little time reading about the exact kind. They were a really good price too about half of the normal cost. I can't set up any more species only tanks right now!
So I feel your pain. Good lesson is don't believe anything they tell you! lol
The waffle plant is nice in a kitchen window vase or as a house or terrarium plant. You could probably grow it out of the back of the tank or in a HOB