There's no harm in doing any other courses alongside Drawabox as long as you have the time to give each one as much attention as they require. That said, you should not attempt to mix-and-match. While I can only speak for Drawabox here, generally speaking courses are designed with intent, to take you through a process where every element builds upon the stuff before it in ways that may not be entirely obvious to you right now. Do not dismantle and reconfigure separate courses, expecting one to fill the resulting gaps in the other.

As discussed in Lesson 0, Drawabox really only focuses on a very limited set of concepts. Confident markmaking, patience and discipline, and above all else, developing a student's spatial reasoning skills. So while you may look at Lesson 3 and believe it to be swappable for anything that talks about plants, or you may think Lesson 5 can be replaced by any animal drawing course, none of these lessons in Drawabox actually are concerned with learning to draw specific kinds of objects. It is rather about looking at the same problem - of understanding how the objects we're drawing exist in 3D space, how to break them down from our references and build them back up on the page - from the different lens that each topic provides.

So to that point, you could ostensibly go and learn how to draw some awesome animals, but that would not pursue the same goal as what Lesson 5 of Drawabox sets out for you.