Taillights really don't need rechargeables so badly, at least not in flash mode. A PB Superflash will last about 100 hours on two AAA's which is a long time. But if you do go rechargeable, you definitely want low self discharge rate batteries, or they'll be dead from self-discharge long before they're dead from actual use.

I've seen some 18650 lights that also include an adapter that will take 3xAAA instead of an 18650. Fine, but the 18650 has about triple the energy of the 3xAAA's ... which makes me never want to use that adapter.
But headlights ... you want rechargeables for them, considering that they have runtimes of less than 10 hours. I'm not a fan of the 3xAAA format, as it's kind of a PITA keeping them charged when your charger will only charge batteries in multiples of two. (I do have one of those charges that charges each individually, but not everybody does.)

Considering that AA's have about triple the capacity of AAA's at the same price, I prefer things that use AA's to AAA's. But most tail lights use so little power it's not a big deal -- but for headlights it is.

18650 chargers? I've got the DX $12 one, it works great. For AA and AAA's, I've got a La Crosse BC-700 and BC-900. Both work nicely.