[ With the images provided above we can clearly see that Lucifer - according to the tail length canon - is stronger due to the length of his tail. And according to the tail length canon the issue of Rin being stronger then Amaimon is also shown above because Amaimon's tail is only slightly longer then his own legs. Rin's is longer then his legs plus add a few inches. And according to this chart for the heights to match with the tails to body ratio could be possibly. Seeing as Amaimon is taller then Rin - but yet Lucifer is taller then Samael but yet the length of his tail is incredibly long. Which means that the height to tail is not including in this aspects. ]

[ The tail of Samael is not yet canon or has been hinted yet to the viewers but what has been canon is the tail color. The tails match the base color of the demon's hair. Amaimon's tail is mostly green just like his hair. Rin's is a dark blue almost black - much like the hues of his hair. Taking this into consideration that means Lucifer's tail is blonde and Samael's is a form of purple shading that's maybe has some hues of being a lighter coloration like the stray hair. But that has not been shown into consideration (See Amaimon's tail). Instead Amaimon's tail scheme matches that more of Behemoth then anything else. Amaimon's demon form even has the same symbol that his familiar has only instead of a lighter green hue it is that of red which matches the color of the familiar's horns - See image below. ]

[ Even with this stated - the white can say mostly comes from his hamster form. BUT his hamster form takes no account for the demonic features Amaimon has. Instead the features mostly come from in tune of the Behemoth. So is it safe to say that the familiar takes tune more for the demon's full demonic side and tail then it does anything else? If so - does it make even more sense that Kuro (being Rin's familiar) has a similar tail then that as Rin's? Rin has a feline like tail same as his feline like familiar. Not saying that - take in nature that Mephisto mentions that he is based semi-from a bird god in native myth. Aka the Raven. And that his magic normal deals with avian like things (aka his bat looking parasol). Then would it be safe to say that instead of his tail looking that of a fluffy animal (much like to say looks similar to Rin's or Lucifer's) that instead it looks more sliming to mimic that of what his familiar is said to be? Which is fan favorite it being a form of demonic avian. Whose to say it's not a demonic avian. Maybe his canine form is just that he prefers. Looking at the myth of Mephistopheles - then yes, the demon from Ao no Exorcist does in fact have wings. So why not a slimming tail for aerodynamics to match? It would only make modest sense. ]