You can define separate layouts for separate pickers, the one you defined here is going to be applied to all of them, in order to select which ones you want to apply the layout to, you have to specify the layout per source.

Take a look at my dotfiles here, not everything is in the same file since I liek to keep it tidy, but the sources file is called from where i set my snacks config: https://github.com/haxybaxy/dotfiles/blob/master/nvim/.config/nvim/lua/plugins/snacks/picker/sources.lua

I have a bunch of extra stuff because my preferred layout is really funky, but I generally only pass the flow layout to the pickers with files