Regarding the KHE Prism cranks, or BMX cranks in general, I have a 68mm BB shell and only had a single issue which was the internal axle spacer being too long. All I did was cut it down by 5mm and it fit like a dream. That's only because I don't have any spacers for the BB.
As for the arm length, they are 175mm and should be perfectly fine. Even the aforementioned Arcade cranks are only available in 175mm and many riders rock them.
For gear ratios, I'm currently in the process of putting a 16t rear sprocket on, 22t front and a double chain tensioner with enough slack so that I can change out to a 25t or maybe even a 28t front chainring. Should be an interesting experiment. This means I can commute with one set, and swap out for playtime quite easily (in theory).