If the NiMH batteries are up to what the web vendors claim, they'll probably take about twice as long to charge as well. The Stinger charger is quite capable of handling NiMH batteries so it shouldn't be a problem; it will just take longer.
It should also be noted that unlike other Streamlight OEM batteries, their Stinger battery isn't priced much higher than some of the Chinese imports, so that is something that you might want to consider as well for peace of mind.
Fox Intl used to have great prices on SL20, SL20XP/UltraStinger, and Stinger Chinese batteries, but it looks like they're not offering those anymore on their website anymore. I seem to remember that the Stinger battery was only $5.95.
I got a used
Streamlight Stinger for free, but it has no battery.
What's the best replacement out there at a good price?
I've seen a few on ebay that say they are 2x longer lasting, but are they any good?
Thanks