I've grown plants alongside my cannabis over the years. I generally start from seed, due to the whole fungal/pest issues mentioned. I currently have various herbs in my veg cab, along with a nice pepper plant. The dwarf jalepeno plant just sprouted a few days ago.
It's the pests thing which prevents me from bringing plants in from outside. You can treat ornamentals with all kinds of nasty stuff and they'll be clean, it's just not my cup of tea.
There are a lot of plants which require a lot less light than cannabis, which makes a lot of spaces in grow rooms to put plants.
I don't keep any in the flower room though.