Yes the PT16 fits 18350 cells that are UNprotected. Protected 18350 cells do not fit, they are too long.
On left is an 18350 rated 750 mAh, on right is a Protected 16340 rated 700 mAh. Note they are the same length. The 18350 has 7% more capacity, so 7% longer runtime, IF you dont use Turbo. So if runtime on a given lumen level was 4 hours w 16340, you would get an extra 17 minutes with the higher capacity 18350.
as you can see, the PT18 head, fits the PT16 battery tube. This is true for all the PT series, the heads lego, to any of the other battery tubes.
fwiw, I just did a test of the two batteries pictured, fully charged, initial lumens on Turbo
450 lumens from the 18350
456 lumens from the 16340
note this light is modified to N219b 4000k 9050, so its output is less than factory spec using stock Low CRI XP-G2
its a homebrew meter, so the numbers are mainly only relevant for comparison to each other.. same light, same meter, only the battery is different
choice of battery is personal preference
unprotected batteries wont suddently leave you in the dark, but if you fall asleep with the light on, the PT16 can overdischarge them.
protected batteries wont be overdischarged, but they can turn off suddenly, and that can be inconvenient if you aren't carrying a spare.
IMR tolerates overdischarge better than ICR
there are IMR cells available rated for 1000mAh, whereas ICR tend to fall in the 700mAh range..
so potentially, IMR can give longer runtime, albeit unprotected