Dahly if you use biospira you can add your Multis immediately. I am so anxious for you to get them, we just love ours so much.
I haven't had any experience with argonite sand and when I did a search it kept refering to reef tanks,You just have to make sure it's not rough as they pick it up in their mouths. so am interested in seeing how it looks. I have Sahara Sand by carib sea, it is a cichlid sand, has a mixture of black and white grains.
As for aquascaping it, No matter what you do they will move the sand around in ways you can't even imagine. They pick up mouth fulls of sand and spit it out, they spit it in piles as you can see in my picture, they spit it at each other. Alot of times they will dig pits and make the shells they are going to live in slide down into these pits and heaven help ANY grain of sand that has the audacity to slide back down. They will also bury any shells they aren't using. so you need about 2 shells for each fish as they like choices. One of ours kept going in and out of every shell when we first got them as juvies and my husband said it had to be a female because she was house shopping and as it turned out he was right

I don't have alot of luck keeping plants except the anubias I have attached to a piece of Lava rock. When I plant anything else they work like demons to cover them up. I have seen others have good luck with plants, maybe mine are just strange.
The lace rock would be pretty with Java Fern or Anubias attached to it. I love rocks

Well enough. Have fun

Carol
PS
Here is a picture of a pit, you can actually see the glass bottom. Do not like the pic, everytime I take a pic of this tank the pics are blue, think it's the light.