Learn Spanish (Latin America): El sustrato es igual de importante que la grava. - The substrate is just as important as the gravel.

Both phrases are perfectly correct and mean the same thing. Using igual de followed by an adjective and que translates slightly closer to just as important as, emphasizing that the level of importance is exactly equal. Tan importante como is simply as important as.

Is it ever correct to mix them and say igual de importante como?

No, and this is a very common trap for English speakers! You must keep the matching pairs intact. It is always igual de paired with que, and tan paired with como.

If the subject were plural, would igual change to iguales?

No, in the specific comparison structure igual de, the word igual acts as an adverb meaning equally, so it never changes form. If we were talking about plural subjects, only the adjective would change to match. For example: Los sustratos son igual de importantes que la grava.

What exactly are sustrato and grava referring to here?

Sustrato translates to substrate, which is the base material that plants grow in or organisms live on, like the nutrient soil in a terrarium or the bottom layer of an aquarium. Grava simply means gravel. This sentence is likely comparing base materials for a fish tank or an indoor garden setup.