you can thread on the cog, but you will need a special tool to tighten the lockring. the tool that works for 9 speed dura-ace bottom bracket lockrings will work with most cog lock rings (at least it works with Phil Wood lockrings...)
For fixies and most bikes, you can get by with a few well-chosen tools. For the tinkering you're talking about, chain whips and a lockring tool are essential, as is a 15mm box wriench for the track nuts. While my tool kit has expanded over the years, the ones I go to most often are:
Allen wrench set 3mm - 10mm
BB removal tool
Headset flat wrenches (now obsolete since I have no bikes with 1" threaded units)
If you factor in a nice Park workstand, you can easily spend several hundred dollars on tools. But since fixies are so simple, there's little that will need much attention, so the once-in-awhile adjustments at the shop may be cheaper in the short-term than buying the tools. But if you're looking at it over many years and many bikes, I think a well-stocked tool box a wise investment, and not prohibitively expensive. YMMV, of course.
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