Ok, let's see here.
I basically have two choices--I can keep it in the garage where the temp tends to fluctuate but will probably (asssuming the weather stays relatively normal) sit between 45 and 58 degrees, or leave it inside at about 66. Or, for a third option I could start it inside and move it back to the garage. Does the secondary temperature really matter?
Yes, secondary temperatures do matter. They matter only in the sense that your beer's flavor will never benefit from any fluctuation of temperatures. It is best to maintain a constant temperature when aging. You should be able to relax a bit, however, since all of the temps you indicate above are fairly low. The worst thing that could happen is if your beer gets really warm, etc. If I were you, I would leave it inside at 66 degrees for the aging. Since your primary fermentation was at cooler temps, you will not develop any esters in the secondary b/c your yeast are primarily dormant and not doing the work of vigorous fermentation any longer. Secondary temp only matters in the sense of "taking care" of the beer. In other words, barring radical
temp shifts, you should not pick up off-flavors to a great degree.
Also, how long should I leave it in the secondary? I've read that a long secondary is useful for developing flavor in higher gravity ales--or would I be better off with a two-week secondary and then letting it age in the bottle? I don't want to crash out the yeast & make it hard to carbonate. On the other hand, I don't really have much fridge space, so I can't "cold-condition" in a true sense.
Leave it in secondary until it is clear and all sediment has fallen out of solution. There are not precise rules, b/c this is one of those brewing "arts." When you see the beer "brighten," then you can move it out of the clearing tank (which is all you are really doing with a secondary phase anyhow).
As for this being a "higher gravity ale," at 1.060 this is not necessarily a high enough gravity to treat like a barley wine or something. While it may take a few more weeks to mature, you won't need any extended aging for a 1.060 brew. I suggest bottling when the beer is clear and aging in the bottle. If you do more "bulk-aging" in the secondary, you will not be hurting the ale at all, but you will need to allow plenty of bottle aging as well. The two aging methods are not interchangeable as the aging with CO2 is distinct from aging off of CO2.
I guess this is the point I am making - if you don't mind waiting a few more weeks than you were planning for the final product and you are really proud of it, bulk age for a long time in secondary and then bottle condition for a long time as well. Otherwise, you only need to leave it in the secondary until the beer is clear.
One more thing - cold conditioning is unnecessary for an ale, but it will give your beer a smoother, crisper, more lager-like taste.