I'm really late to the show here, but actually roaches don't need much protein. I'm parroting information from a professor that has been posting his research on roaches since 1966, so I consider him a better authority than anybody else I've heard of. Below is an exact quote.
The 4% protein diet should satisfy all stages (nymphs and adults). You can produce some happy medium with a minimum of work by allowing them to choose their own diet but forcing them to get some of their moisture from vegetables such as carrots and potatoes that provide more filler that is low in protein and provide a restricted amount of the ~20% protein from readily available dog/rat chow.
So that should answer the question about roaches eating potatoes as well as protein requirements.
I have a fairly new (about 2 months old) Dubia colony and between oranges and carrots, they will usually choose carrots. Although some days when I put both in they ignore the orange slices and devour the carrots, and other days I'll come back and the carrot will barely be touched and the orange slices are gutted to absolutely nothing. I also give apple cores, oats, about any fruit/veggie that is left over from dinner. My colony is too small to have to worry with DIY roach chow, so until my colony grows a bit I am feeding the Lugarti's Premium Dubia Diet and what remains of my Fluker Farms high calcium cricket food, and letting them decide which they need. Many roaches will avoid some foods and eat others when given a choice depending on their nutritional requirements, although as the professor said, roaches can eat a totally un-nutritious diet and still thrive!