The first major parameter to recon with is light intensity... Most plants if supplied with ample CO² don't care much if it is high or medium light. Except the slow growing plants that are rather requiring shady conditions by nature e.g. Anubias, ferns and buce etc. Not saying these plant sp. can't grow in high light conditions, they do, but with a rather narrow error margine. Than if you cannot provide them a somewhat shaded spot in the scape you are at risk.
Than it all comes down to, knowing the plants.. Meaning, slow or fast growing, potential size in given conditions.. Take for example Echinodorus tenellus tenellus, this plant will grow significantly smaller in lower light intensities and no CO². Than depending on the dimension of the scape and given conditions this could rather be a midground than a foreground plant. Some stemplants can grow in different sizes from mid to background type. Some stemplants tend to lose lower leaves due to growth density and or light insufficiency, this are plants you definitively rather want as much as possible in the background, with a midground plant in front of it to cover the leavless stems at the back.
And all that again comes down to experience and this takes more than a few scapes. It is always ok to look online for inspiration i try to copy scapes you like. A pit fall in this is often the parameters may differ significantly. Most plants don't care much for these parameters and grow anyways but this can significantly cause different grow behaivor in size and speed. Here you could take for axample a valis, usualy described as a pant growing 30 to >60cm, Larger than 60 means it also can easily grow over 120cm in lenght.
Experience comes with a lot of trail and error.. Another example i experienced my self also in high tech situation, some plants i cannot grow for what ever reason. I have no idea why, but i always failed to grow Staurogyne repens, pogostemon helferi, Bolbitis hetroclita.. I tried a few times under different fert regimes and over again they always die on me.. Must be in my tapwater parameters i have no other explanation i dunno, it's what i get from the tap i can't change. Thus how hard i want, i probably will never shine with a scape filled with these plants.
Anther plant i cannot grow in low tech condition is Cryptocoryne spiralis var, shola, i love this plant to death and it always dies on me.
Search youtube for The Green Machine, James Findley made quite a few making of videos. Very inspirational to watch and in 90% of his scapes he finaly decided to swap some plants, because they just didn't grow the way he previously thought they would. And that's a professional scaper.