Make sure the macerate or very finely mince the habanero if you use it; otherwise some bits will burn like Hell, and other will not be hot at all. I find the heat from the habanero more pleasant than from cayenne pepper, and so I highly recommend using them if you have them. One of the nice things is that the fat from the avacados seems to bind well with the capsaicin resulting in a quick burst of heat that fades very quickly and does not become overpowering over time. I once tried this with two peppers and it was not a good idea: the heat dominated and drowned out a lot of the other flavors.
As for the salt you can use more or less. The recipe I started with used a teaspoon of salt, but the local burrito place guacamole was rather much tastier and noticeably saltier so I tried a tablespoon instead and was pleased with the result.