You may be able to just lay some along the substrate, but it will probably take a bit of time for the plants to anchor and orient themselves.
maybe try getting a couple different varieties of each, just for some attempt at diversity.
On the upside, it shouldn't take much maintenance...
On top of tieing them to the hardscape you can place them at the base of your hardscape - it looks great. To do this I went out to my yard and washed/boiled a few pieces of gravel and tied smaller pieces of Anubias to them and then placed them around my driftwood. It all grew in and formed a nice bushy base around my hardscape. Also it'll grow into your substrate after a couple of weeks - just make sure to put some ferts in the water column.
Since those plants look terrific attached to a strong hard scape it would be a bit of a waste to plant them in substrate. Get a substantial branch you like that and a lot more rocks than you think you need. Put rocks under and around the branch as it is laid on its side and plant anubias among them and put java ferns at major branchings on the wood. ADA has a lot of this sort of thing on their youtube channel. Often plus foreground and background stems but it looks great without. I would use sand and more small rocks where there isn't wood/rocks.
As long as you like the rocks it is fine, you don't need to buy the expensive bonsai/aquarium ones. The first ones I used were dug up in my yard over 20 years and finally I want them! They are cobbles with smoothed edges but with a lot of texture. Just try to get ones that are more or less the same sort, not black and white mixed.
Right now my plants are tie onto lava rock.
I want to remove the soil under my sand since there really no need for soil,
A stupid thing I did. Also I'm still hunting down seller that will sell 30-50 anubias at a discount.