Substrate, How Much Needed For Live Plants

Sand or small grained gravel is best for plants; larger grained gravel is not as good. I know you said 'small pea sized gravel' but does that mean pea, petit pois or smaller?

2 inches/5 cm is usually recommended for plants.



I get round plants not staying planted by having plants that can be attached to decor or floating 

You can also tie bare root plants to the ceramic rings that are often found at the bottom of the plant pot, then bury these in the substrate (ceramic ring on top of the roots). I've done this recently with an echinodorus that I planted in a spot where Bertie the BN apparently likes to dig now 

As for substrate depth, around 4cm, though it probably gives the information on the pack (if you can find a pack of what you already have in a shop and take a look). Push the plants in deep, and perhaps put a stone on them so they are not disturbed before they root, which I've also done to discourage Bertie from digging up that plant.