Your best bet is really to hit Google for a bit and just do some reading. You are asking perfectly valid questions, but they are questions that have been asked many times before, so all the info is out there already.
And really you just need to brush up on some of the basics. Get a grasp of general battery terminology and specs and the rest is mostly easy, of which you'll likely be able to answer a lot of it yourself.
Yes. You need to find a RCR123 that is 3.0 volts. There is confusion because not just differences in chemistry, there are some cells that are RCRs that are 3.7 volts or 3.0 volts. To add to the confusion, I have even seen some labeled 3.6 volts. Soshine make some decent 3.0 RCRs. Also know, non-rechargeable CR123s (like a Surefire 3.0 volt 123) are commonly referred to as Primary Cells. To add to the confusion, some of these batteries are called 16340s. These can be called RCR123s too. Then there are Protected and non Protected, flat top and button top. Lol
This is great information! So, is it safe to say 16340 is simply an alternate name for an RCR123?
The easy thing to start with.
Most Li-ion have a 'rested' voltage of 4.2v and a nominal voltage of 3.6-3.7v depending on where you read it.
It's important to know that a battery when charged and sitting doing nothing will put out a different voltage to when it is under load. This is generally why you may see the same battery listed with multiple voltages.
But just take it that most Li-ion will be 4.2v rested.
There are 4.35v Li-ion, but they will work just like a 4.2v with lower capacity if only charged to 4.2v
There are many types and chemistries of Li-ion, but ignore this until you get and understanding of the basics.
LiFePo4 is something different. And you shouldn't see it being called Li-ion. It has a lower voltage of 3.0v nominal and 3.6v resting. These are the closest thing to a CR123a in terms of voltage output and can normally be used as a direct replacement. While Li-ion is always higher voltage than CR123a's and should only be used if the light in question specifically claims to handle the voltage.
Too high a voltage will simply fry electronics.
You MUST know the rated specs for a torch before putting a battery in it.
I would for the most part ignore mAh until you are happy with the above. mAh is simply capacity, e.g. just like how many gallons your fuel tank on your car can hold. It generally has little bearing on performance or compatibility.
Just as a heads up, the biggest capacity doesn't always mean the best performing, just as cars with large fuel tanks aren't always the most fun or powerful cars available.
As for sizing. Li-ion can be had in multiple sizes, e.g.
18650
18 = the diameter in mm
65 = the length in mm
0 = is a designation, however it is wrongly assumed to mean a round cylindrical battery. Which actually works out fairly accurately in the torch world. Just remember that Li-ion extends far beyond flashlights.
Common torch sizes:
10440 (AAA sized)
14500 (AA sized)
16340 (CR123a sized, sometimes branded as RCR123 for rechargeables)
18350
16650
18650
26650
Welcome to the forum.
