I have a g19, will probably wear a red dot in the future.

I can't find an answer to this. I want to buy a holster, but not have to buy a new one when my glock wears an rmr. Does the 6354do holsters work well (or at all)  when the gun doesn't have a red dot on it?

Yes they can be used with or without optic

Will it be secure in there or flop around like some other holsters if you don't have a light?

Double tap

It will be secure in a 17 holster or 34 holster as well. With or without a light. If you might get an extended threaded barrel, get a 17 holster for the extra length.

It's an ALS holster, not an SLS holster. It locks off the slide not just a flip up retaining loop. It is only a tiny bit harder to reholster without a light. The dot sight makes no difference.

That said, I use mine with a Deltapoint Pro, as I would never put an RMR on a gun. My 19 with no Deltapoint Pro fits and locks up fine.

I have 3 of the 6354DO and one of the newer RDS Safariland holsters with hood. The 6354 DO let's dust and drizzle cover your lens. It's a real pain to have to wipe clean all the time. If you don't have to have camo, I'd consider a RDS variety of Safariland with ALS or ALS/SLS combo. Also, make sure you get a thumb safety for the SLS so you don't have your rifle swing over and the grip push your release button on the ALS and allow your pistol to pop out the holster, land upside down, and destroy your optic.

Looks like there is also a 6354RDS so it has hood and QLS fork, but only in black STX and STX basket weave so far. The RDS hood is the way to go.

https://holsterops.com/collections/optic-sight-holsters/products/safariland-6354rds-als-optic-holster-li-duty-with-qls19

Hopefully we get FDE too soon.