There are bonding requirements for CSST developed to eliminate issues with lightning.

CSST can pass through the floor, it may need some protection.

If you are talking about a flex whip it should not pass through the floor.
Bring the iron pipe up above the floor and then connect rthe flex line.

In my area, the longest run of flex you can have between the black iron pipe and the stove is 6', there must be a readily accessible gas shutoff where these two join, and the shutoff must be in the same room as the stove. I agree that the best thing to do is to extend the black iron pipe to a location near the stove: this will minimize potential problems with the more fragile flex getting damaged at some point. You should also check with your local code people to see what is to code in your area. When it comes to gas, it's wise to adhere to code requirements.

You have to check with your local code. In my area we are not allowed to have any flex line. We have to use BIP or brass pipe all the way to the appliance regulator.

"In my area, the longest run of flex you can have between the black iron pipe and the stove is 6', "

Short flex connections are not the same as CSST.

In areas that allow CSST it can be used for almost the entire job.

Thanks for your replies. After consulting with 2 licensed plumbers (gas makes me a little nervous), it turns out that the best position to put the line is actually right through the air return joist bay, and flex line is apparently not allowed for that, so it will have to be the hard black pipe, regardless.

Thanks again.

joe71-
Thanks for the followup. It's always good to know how these things turn out.