Did anyone tell you the dangers of those plastic skulls? They can get stuck in them and you really really do not want to have to break them out.
Reducing nippiness just takes time and patience. Assertiveness when taking your snake out of their cage and cupping them from the top and bottom will reduce defensiveness. Make noise like tapping on the enclosure a few time or the hide so they learn you are coming in to handle. Never tap when feeding so they don't associate with that.