Oh boy. This could take a while, you settled in? Okay, let's go...
Sumo isn't really Jujutsu, although they are both Japanese fighting methods. The origins of Sumo are, as in many other things Japanese, shrouded in legend. The story tells of two gods who battled (named Kuyehaya and Nomi no Sukune) who were reknowned for their strength. In order to know who the strongest was, they were set to engage in a match. Both kicked at each other, with Nomi no Sukune breaking Kuyehaya's ribs, then, as Kuyehaya fell, Nomi no Sukune kicked him in the groin, killing him. In Japanese records, it is said that this happened in the 7th Century, and is the first record of combative techniques. Sumo's rituals trace their origins to this story.
As for what Sumo is, the techniques are rather simple, involving pushing, pulling, slapping, and a great deal of leverage, but little else, so it really doesn't qualify for what has become known as jujutsu.
The concept of Jujutsu itself is rather complicated as well. It has become a gneneric term, but was by no means universally used, or even the most popular historically. It could be used to describe unarmed combat methods, lightly armed combat methods, striking methods, grappling methods, methods in armour, methods when not in armour (suhada), and much more. And to add to that, different schools and systems would have different terms used for these concepts; for example terms used included Wada, Wa, Wajutsu, Taijutsu, Koppo, Kosshijutsu, Taijutsu, Yoroi Kumiuchi, Hada, Yawara, Te, Dakentaijutsu, Torite, Koshi no Mawari, Aikijutsu, Judo, Kogusoku, Kowami, Hakuda, Kenpo, Shubaku, and more. And just because two schools used the same term didn't mean that there was any relation between them, or any similarities in their methods. Then you have schools such as the Yagyu Shingan Ryu Heiho who have different terms relating to different aspects of their syllabus (Suburi, Torite no jutsu, Totte no Jutsu, Kogusoku Totte, and Gyoi Dori - I'm personally rather fascinated with the Gyoi Dori section, as it deals with protecting others, something few other arts look to), and that is before you even look at their weapon sections! And on that topic, a Jujutsu school, classically speaking, will often have weapon sections as well, such as Sekiguchi Ryu and Takenouchi Ryu, both well known for their Jujutsu, but also having very skillful weaponry teachings as well.
When it comes to "uniform", that appears to generally come from the clothing of the day, as well as the uses of the school itself. Many origins for the "traditional" gi talk about it being the undergarments normally worn beneath a Kimono, or beneath Yoroi (armour), and this shows the gi being basically the equivalent of sweat-clothes that you may wear to the gym, essentially just what you wear so you don't get your good stuff ruined.
So was it always worn? Depends on the school. Some wear Samue, a basic utilitarian workwear, some wear Hakama, a more formal pant similar to a split skirt, some go for a gi, addressed above. Kano Sensei did not "create" it. He did establish it as the worn uniform for his schools, but it is rather similar to what he had worn in the Kito Ryu and Tenshin Shinyo Ryu beforehand. It is by no means a universal uniform, although it is relatively common. Each schools dress is decided by the head of the school itself, that's the beginning and end of it.
Does that help at all?
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