Will soldiers / town guard wear uniforms and stuff, which involve using specific weapons or armor? If so, will adventurers working for a town have to wear it, and will be able to identify soldiers from other towns?
They currently do, whenever you look at the clothes (usually helmets and armor) of town guards, they'll tend to have their civilization or entity symbol embroidered on them. Sometimes is not as easy, but there should be some kind of quick way to check it out, as soon as it turns relevant.
What exactly does the Tavern arc consist in, what modes will it primarily affect and is there a planned timeline for it? Will its development cycle be as big of an undertaking as the current one?
Though there's no actual Tavern Arc in the
Dev_Single page (which is where I assume you got your doubts about the existence of that specific arc), you might also take that the avaliable development list is outdated and inaccurate, in terms of Toady's ongoing plan.
The Tavern Arc, as we've heard, meshes a lot of stuff from the Diplomacy Arc, the Crime and Punishment Arc and a lot more. It might allow your fortress to have foreign denizens and floating populations, it will be a source of valuable information and rumours as well, it might help the economy and also provide entertainment for and by idle dwarves. Expect drunk fights, adventurers gathering, thievery and, why not, a disease vector.
Arcs, as they're being developed right now, are not lineal and self-contained, rather branching themselves while overlapping their checks and goals.
Also, see Manveru's ninja post.