I would tend to think this is more a case of confirmation bias than anything else when trying to correlate red hair to temper (even your question cites a counter case with "Allie"). Numerous studies have been made between hair colour and various other traits, and they all pretty much come up with poor correlations in terms of actual personality. The abstract on this paper states (emphasis mine):
A collection of studies beginning with one that looked at correlations with descriptive terms chosen by young males for blondes, brunettes and red heads and by the females themselves for Eysenck's EPQ-R personality measures. The descriptives were chosen in an absolutely stereotypical way. There was an apparent association between extraversion and blondeness, lie scale (social conformity) with dark hair and psychoticism (social non-conformity) with having red hair. The follow-up looked at stereotypes of women about women, which largely confirmed those of the men in the earlier study. The next study looked at how much or if at all dying hair to a different colour changes personality. Using the EPQ-R again, only those women who dyed blonde showed a significant change in personality; however, with reference to the first study they did not become extravert or more extravert but became significantly more psychotic (socially non-conformist). Theirs was a banner of rebellion!
In reading this paper, and a few others, it generally appears more the EXPECTATION of how one behaves that drives the actual behaviour based on the hair colour.
Amongst males in the UK (CEO level), it was again concluded that people's perceptions had a greater corollary than actual personality (PDF).
Numerous "twin studies" do give credence to the "nature" hypothesis of how people tend to develop, however, there seems to be very little actual study on if there are actual correlations between the personality of the person and their hair colour.
Keep in mind, most of these studies are Euro-centric. For instance, in Chinese culture, red is considered a bright and happy colour. So anyone in china that is fortunate to have red hair (very rare mind you), is considered a fortunate and happy person.
As with external traits, and personality, there is no single catch all. We still do not understand how genetics play into developing a person's overall behaviour. What switches are involved that, for instance makes a person aggressive, temperamental, etc. and how they are tied to other genes. Maybe you would have better luck at getting a deeper understanding at the Biology StackExchange?