I use rechargeables in everything I can, mostly 18650s and 123s, the new Li's and poly Li's stay charged for years, don't care about the cold, no memory effect and cheaper than dirt if you get them from China.
Just make sure the regulator in the flashlight can deal with them, they give a little higher voltage, some are up to 4,7V.
Length and width can also vary a little.

On the backside, Li batts can give you a nasty fire...