It's possible to shorten cables, but not easy. The solder used isn't normal electrical stuff, that's too soft to hold much pull. Maybe for a throttle cable, but definitely no good for a brake or clutch cable. At the OEM factory (I watched at a small cable works in Osaka) they slip the nub on, crush the wire end with pliers to open the cable strands a bit, slide the nub down in place, then dip just the tip in hot liquid flux and then a very hot solder pot. I don't know what the metal mix they use is, but it is very hot - not as hot as you would need for silver solder but hotter than the 600F or so needed for electrical solder.
The only stuff I have found for this job is brazing rod or silver solder (probably no actual silver in it): both are harder and melt at a much higher temperature (dull red glow) than electrical or the mystery OEM solder, beyond the range of a soldering iron or propane torch. You need a gas + oxygen torch, a proper oxy-acetylene rig is good or you can get a home kit with small cans of some gas (not acetylene) and oxygen that works pretty good.
One problem is the old cable wire being soaked in lube and probably rusting a bit over the years. That makes it rather difficult to get clean enough to "tin up" with solder. Using an aggressive flux and lots of heat will usually work, but using a new core wire is the best bet. One tends to either overheat and weaken the core, or wick solder way up stiffening the cord for the end inch where it really needs to be flexible.
You can buy the inner core wire and the various end nubs (and the outer tube and sleeves too), if you have the torch and stuff to do silver soldering then making your own cables is easy. The only trick is to avoid having the solder wick up and stiffen the end, that's a matter of controlling the heat so only the nub and last few mm of cable core get hot enough to melt the solder.
