Yes. The length of spoke needed depends on the hub, lacing and rim. The variance can be several millimeters between different combinations of hubs and rims. They will be in a range for a given wheel size but they vary a lot in that range.

I would suggest that you use the wheels you have but you have them tuned before you go. Get several replacement spokes (no more than 6) of the proper length and zip-tie them under the rear rack. You may also (or instead of) get a Fiberfix spoke.

Don't take this the wrong way, but given the nature of your questions, I assume you don't have much wheel building experience. You should have someone show you how to replace a spoke on the driveside of a wheel (most likely spokes to break). It's something of an involved process and not something you should learn "on the road", figuratively or even literally.