The drums typically are the loudest sound in hip hop, but not so much in Rock, and other genres. I will assume you are speaking on hip hop where typically the drum sounds are the loudest. Typically I find that the loudest sound in a hip hop/rap drum mix is the snare drum. Followed by the kick. Everything else typically goes under the drums. as to rather you should layer, it really depends on the sound you currently have, and the sound by which you are going for. There isn't a right or wrong nor rules. Layering is just a technique to achieve a goal. If you feel like you need to add something to a sample/sound that the original sound doesn't have. In that case you just have to figure out what you want to get out of the sound and eq accordingly. As Wizworld said above me there is a lot to consider when it comes to sound design itself.
but Keep it simple, yeah there's a lot of techniques and things to consider, but in the end just worry about making the sound sound good to your ears, Don't try to learn everything at once otherwise you could go down a rabbit whole, and never find the answers you are looking for. Techniques may change, but the goal will always be to get the best sound you can from what you have and the techniques you know. Learn a little everyday to build your knowledge, but don't stress over it. Learn enough to get you to point B, and as you go forward you will learn more in all of those areas that will quicken your sound design in the future.