-6 is kindof just a guideline, it can really be anything as long as nothing is hitting 0db when it goes into mastering.
Well, there is such a thing as too low because of noise floor, but for all intents and purposes you'd have to go out of your way to make your stuff low volume to make this an issue.
Incorrect, there is no noise floor in a digital system
There is, it's just ridiculously low with floating point quantization. But any quantization introduces noise, and it's right there when you convert to a fixed point file like 24 bit wav.
If it's purely digital. Not everyone makes purely digital stuff though, and you can get into trouble if you don't gain stage properly.
Edit: Actually, looking into it it seems that there IS a purely digital noise floor, it's just absolutely absurdly low, unless I'm misunderstanding something, which is totally possible.