From what I've read, a device that needs power over a long period (i.e., smoke detector) is better off with a regular battery. I think the exception would be a lithium ion rechargeable, but check with a smoke detector expert on lithium ion.
Anyhow, in general, rechargeable batteries (i.e., NiMH) are better for devices that need power for a shorter period, such as a wireless mic, an electronic toy, etc. Why? Because NiMH rechargeable batteries discharge energy more rapidly versus normal batteries. They just won't last as long as regular batteries.
Besides that, when a NiMH rechargeable gets close to the end of its charge, it can drop its power rapidly. That's less warning time for a smoke detector.
So in a way, it's a blessing that the Tenergy batteries were a bit small for your smoke detector. Did you use them in a different device?
Also, did you contact Tenergy to ask why they're small? That makes me curious, too.