Try to avoid too many battery types but you don't have to be super-strict about it. I'm guessing your Arc is an AAA, great little light. The battery lasts quite a long time and spares take up very little space, so if you have mostly AA stuff, but your Arc AAA and a spare AAA or two, it's no big deal.

If you want to go all AAA, that's doable (Arc AAA flashlight, PT Rage brighter backup, Garmin Geko GPS, any number of AAA-powered radios and FRS/GMRS handhelds, etc). AA is even easier and AA's last longer, but AA stuff is bigger.

Used to be that all the cool flashlights used CR123A's, but not so much any more. The Fenix L1P uses a single AA and does about the same thing that most of the 123-powered LS lights do. It's the most popular light on CPF right now (CPF= www.candlepowerforums.com , where the flashlight buffs hang out). So now I think you can avoid 123's. The one thing I know that uses them that there's not an obvious substitute for is the MSR Miox water purifier, but not many people use those.

Worst of all is proprietary battery packs like most cell phones use. A spare is a good thing. There's also a gizmo available that lets you power a cell phone from an AA, at least for a while.