"Mi ĉiam pakas kombilon kaj broson."
Translation:I always pack a comb and a brush.
A "brosilo" would be a tool for doing something with a brush. Possibly in a factory: Jen la brosilo, kiu kreas brosojn.
The key thing here is your comment about the kinds of roots.
bros/o is a noun root. komb/i is a verb root. bel/a is an adjective root.
Depending on what kind of root you start with, the rules about how you use them are different.
In most cases it's self evident which category a root is in. "Dom/o" is clearly a noun, "hav/i" is clearly a verb, "inteligent/a" is clearly an adjective.
But there are some roots that aren't as transparent, and those you just have to learn.