Earlier quoted context omitted.
But so long as the bash scripts have the shebang, and all should, things should work properly in theory with fish able to load bash when needed, surely? Or is that another in a long line of "yes, in theory, but…"s?
Or you can insulate yourself from bad scripts without the #! by just calling bash then running your script. In my case, since I run so few bash scripts, I use fish (and ksh on BSDs) and call sh when needed.
That works when you call the scripts yourself, not so much it's scripts calling other scripts.