Just getting started, but I intend to do more like this: https://www.flickr.com/photos/trevarthan/14562711778/
Is there a rule about what mm length is good for panos? 45mm and up? or can you use wides too?
That image may be doable with stitching, but it is a risky case because of the possible impact of wind on the trajectory of the water jets. If there is a little dispersion due to a slight breeze, then it should work. If you have irregular wind patterns, it will probably not work.
I would for sure back it up with a single shot to be sure.
Stitching can be done with any focal length, but be aware that there are limitations to the angular coverage that can be projected to a plane. Let's say it corresponds to a 12 mm non fisheye lens on FF. So if you stitch with a wide lens, let's say a 24mm, then you will only be able to do a 3-4 images stitch in vertical orientation. Beyond that you get into angular coverage that cannot be projected to a plane, which means that you will have to use a cylindrical projection that doesn't protect the linearity of elements, lines will be displayed as curves.
In other words, the longer the focal length, the more images you will be able to cover a given view angle with. The plus side is more resolution, the minus side is more time to capture, more risks if you have moving elements larger than one tile in your composition and a bit more work in post-processing.
I typically stitch with 55mm, 100mm and 180mm lenses depending on the type of scene. For your water jets, I guess that you must have used a 35mm lens for this shot? I would try it with a 50-60mm lens, probably 2 rows of 3 images. Keep the exposure relatively short, no slower than 2-4 seconds.
I would use PTGui to do this stitch. Make sure to output the result as a layered psd file so that you can edit the masks in PS to correct possible small issues.