No issue at all, and if any wires are shorter than needed you can extend them with pigtails. Pigtails do not count in box fill, but that does not mean they may not make the box harder to work in or a tight fit. Most of the time the pigtails are held as short as possible to avoid crowding the box and use the smallest wire nuts allowed for the wire size. It can get to be a real problem to push everything into the box sometimes, especially if there are any dimmers present. Dimmers while much larger than snap switches, still incur the same volume limit in boxes.

In a "normal" or modern 3 way switch circuit, the neutral is wired to the shell of the lamp holders. Through various combinations of wiring that varies depending on where switches and fixtures are in relation to each other, the hot line is switched between two travelers and the hot to the lamp holder is switched between the same two travelers. When the two "hots" are switched to the same traveler you get light. The Carter system, Arkansas 3 way, California 3 way (bunches of names) runs a hot and neutral to each switch. The common is one wire to the light fixture from each switch. Depending on switch positions you have two hots, two neutrals, or one of each to the fixture. The one of each gets you a lit light. The 3 way dimmer at the bottom is only switching one wire to the light from hot to neutral. It will depend on the position of the switch at the top of the stairs whether changing the wire to hot rather than neutral turns the light on or off - - but, if the wire from the switch to the light is shorted to ground anywhere (metal lath would be a good candidate)it will trip the breaker when it becomes hot. The right switch combination results in the dimmer switching the wire to "hot" to turn off the light, that's why the breaker trips when turning the light off So, you probably have 80 year old wiring going bad - could be from a number of things. Unfortunately, the only correct fix is probably to pull a new cable and rewire the circuit. (once new wire is involved they can't put the Carter system back - has to be to existing code rules). That would likely include a home run (to the panel) to accomplish the required grounding. Probably not going to be cheap or easy and will likely require professional wall/ceiling repairs to match existing. If the diagnosis is deteriorating insulation on the wire, you may want to be budgeting for more work soon.

Other than WHITE and GREEN there are NO standard color coding for electrical wiring. It's more important to look at the house wiring and then READ the instructions that came with your dimmer. Equipment grounds are always green or bare. It's safe to always connect these together. White is the grounded (i.e., neutral) conductor USUALLY. However, in older houses it may be a power feed for a switch loop. Two switches controlling the same light are called "three way." One terminal is a common which goes to either the lamp or power. The other two are the "travellers" which go to the other switch. What are the wires connected to the other switch (there should be three wires in addition to the green/bare). Then you'll have to tell us what the model dimmer you bought (or at least read the instructions for yourself). Not all dimmers can be connected to a regular threeway by the way.

I can understand...but as it's only used possiby twice a year for tilling the modification seems more logical than spending countless hours and $'s trying to find the problem in the mess of wiring and safety switches. Guess I'll just have to keep digging.