Which Brett strain to use

I used to make a Christmas stout (Russian Imperial Stout) that I would brew in December and then age until the next year and give out for the next Christmas to a select group of friends. I am going to make it this year, but am thinking about adding some other factors. I was using Molasses to push up the gravity but, thinking of the English root of this style I'm considering Treacle. Also, It will sit in primary for probably two weeks or so, then secondary for a month. In secondary I plant to add wood chips. I'll then use a tertiary fermentation with a split batch. One half will be without Brett and the other half with Brett. The question I have is what strain do I use for the Brett? I am considering Clausensii, but am wondering if someone could give recommendations and considerations for this. I may also add some sourmash to the boil, but am not sure if it would simply be overpowered by the brett.

It depends a lot on what kind of flavors you want. Personally for a RIS I think the best brett flavor contribution would be the funky cherry flavor out of the strain sold as lambicus although other brux strains will give you that same flavor if given enough time. I have a brett beer that's about two years old I brewed with Orval dregs that was barnyard funky the first 18 months and then developed into a really fantastic earthy cherry flavor. You may not want to wait a couple years for the flavor to get there so lambicus would be a better mix.

If you want more of the tropical fruit clausensii or custeranus would be your best bet.

Think about what you want from this beer. Not really sure Brett is going to give you what you want, it can give you some really funky flavors.

Brett ... Sourmash ... do you really know what you want and how to get it. A little sourness goes well with a stout, maybe use a pound of acid malt. A sourmash is really hit or miss, unless you want it to be puckeringly sour.

Brett will not give you any sourness.

I'm seeing the sourmash and the Brett as two different parts of what I hope to be a very complex beer. I have a beer that I make with a sourmash that I absolutely love. I know this will be a good addition to a stout. When I said I wondered if the Brett would overpower it, I meant that I wondered if the Brett additions would simply be so strong that you would never get any of the sourmash.

As for the other points, I am not a big fan of cherries so will likely stay clear of the Lambicus and shoot for the Clausensii. Like I said, I plan to split the batch and finish one on Brett the other without. I expect the Brett part to take two years. I want to save out several from the non-Brett batch and then do comparative tastings for a few years to see how the Brett and non-Brett age. Thanks for the input.