Is it OK to bottle some of my beer in growlers?
That’s what they are for 
Do you mean to bottle condition in them with priming sugar? They may explode as they are not designed for the pressures of bottle conditioning beer.
The standard glass ones I agree. How about some of the nifty new stainless steel ones?
Yup, to condition.
Side question, can you fill the growlers with already conditioned beer and expect the beer to last for a period of time?
I would disagree with this statement.
Growler glass is actually heavier (thicker) then regular one way commercial bottles.
Sticky point is growler cap.
There is couple of different designs.
Some of them better then other.
Your other question:
I fill growlers with CPF and they last at least 1/2 year when kept in fridge.
i don’t think you would have a problem either, however,the glass may be thicker but the radius of a growler is also bigger and the ability of a cylinder to withstand pressure drops as the radius goes up.(and it is the radius squared so it changes quickly)
You will have a problem if you bottle condition, or if the beer gets infected with bacteria or wild yeast. Growlers are not pressure rated like beer bottles. There is a reason Belgian bottles are thicker and ahapws in a certain way for bottle conditioned beers. I have had several growlers of bottle conditioned beer explode over the years. These explosions can be dramatic, leaving nothing but tiny glass fragments and a puddle of beer. You don’t want to be anywhere near one of these things when it blows. It is very dangerous.
CPF?
Counter pressure filler
I’ve filled the swing top, jug handled many times with no ill effects. Mostly when i rack to a 5 gallon corney and have some leftover, I’ll put the excess in a growler and bottle condition. If your using screw top growlers or even the swing tops for that matter, if there’s a concern you can always bleed off the excess pressure.
I can only speak to filling with fermented beer and have had no problems with it keeping for several weeks until consumed…
Dave