1. I don't know if locs can be "too soft" in the sense of being overconditioned, but I do know that if your hair has a softer texture, it can make it more difficult to loc up. Not that it won't loc, just that it will take longer.
2. From what I understand, one of the biggest culprits in loc breakage is overtwisting. I've not heard any specific ingredients that can cause breakage, but I would guess that if an ingredient causes problems for you (general you) with loose hair, it would most likely do the same with locs.
I'm currently a little over 6 months into my loc journey, and still researching and experimenting, so I am certainly no expert, but my hair has sort of become my obsession. I've gotten most of my information from youtubers and bloggers. There are quite a few out there that really know their stuff!
Hope this info helps!