You already span the range 18-250 mm with the lenses that you've already got. They are zoom lenses however and there is a big advantage with prime lenses - their fixed focal length can with some practice be known by heart.

To know how the picture will turn out before even looking through the viewfinder is a huge advantage when already facing a situation that from a photographers point of view can be as short as an blink of an eye. By using a prime lens you can put all your effort into nailing the focus and composition and not having to worry about also getting the focal length right. This can be accomplished with duct tape and a zoom lens, but primes generally have larger apertures which you benefit from both in auto focus properties and low light situations. Also their one focal length only design allows it to be optimized for that focal length and almost certainly will give you higher quality than a similarly priced zoom lens.

The 40 mm pancake lens is an excellent choice in street photography in the sense that you won't look as intimidating using it compared to larger lenses. It's also rather cheap.

You didn't specify which 50 mm you where thinking of, but I assume the 50 mm f/1.8 considering what other gear you're using. I can't say anything else than that it's a great lens that (considering it's price) and that you certainly will make good use of it as a street photographer. As noted below its focusing speed in not the best and it can result in not locking the focus before the opportunity already has already passed.

Otherwise all reasonably fast auto focusing primes in the range around 35-50 mm are great options. You are using a cropped sensor camera though so I would try to lean towards the shorter end of the spectrum. Wide angle lenses (shorter than around 30 mm) are an alternative, but they tend to be rather expensive.