Adding/substituting non-flour ingredients in bread
The big question you have to ask yourself is, "How absorbent is what you want to add?" For instance Chia seeds are not going to affect your hydration much. Those you can probably just add. Corn meal on the other hand will absorb water, but it won't create gluten. You'd want to replace some of the flour with corn meal and perhaps a bit more vital wheat gluten. Wheat bran contains gluten, oat bran does not. Both will be absorbent so should replace flour. Any cereal will be absorbent, the question of gluten will depend upon the grains used. Because you're using a bread maker you're probably not as intimately aware of the hydration of your dough as you would be if you did your kneading by hand. For your next few loaves, take the dough out of the machine when it is done kneading. Give it a few kneads to get a feel for it. You'll feel it if your hydration is off.

You're right in that you want to try adding one ingredient at a time. Also start small, you might just be surprised at how much 1/4 cup of corn meal will affect your loaf.

I find it helpful to look at recipes that include the ingredient I want to add and compare that recipe to similar loaves that don't contain the ingredient. In what other ways are the recipes different?

2 websites come to mind that you might find helpful.

The Fresh Loaf and King Arthur Flour