Hi there... Going to switch from a Motobecane Messenger I've been upgrading throughout the years (but that has always been a tad bit too small for me) to a Soma Rush that I actually managed to try out. Going to move all the components, but I want a threaded fork and I was looking here for ordering... Not 100% sure what size thou'...

The Rush headtube length is 130 (for a 55). I would add a Chris King 2Nut and a Nitto Pearl. Looks like the 170mm would be perfect, but I know that if it's too short I'll be ****ed. Should I go with the 200mm and add spacers if necessary?

Thanks in advance.

Take into account the stack height of the headset.

170-130=40mm

As long as you select a headset with 40mm or less of stack height you should be ok. If the headset is shorter than 40mm, you can add spacers as needed to take up the slack.

Or you could cut the steerer.

Yes, you could. But that would limit your future headset choices to those at or below the new length. At Trek, we cut all our steer tubes for 42mm stack to allow plenty of room for any headset and even customer-added things like brake cable hangers or lamp brackets.