I am no dietary expert, but I would have to say only in small amounts. Remember, shrimp is not a part of their natural diet, so be careful. I don't see anything about shrimp that may be toxic in high levels except maybe mercury, which in high doses can cause kidney and brain malfunctions, or death. Seafood is tested for toxic mercury levels on a regular basis. Are you buying it from the store, or fresh caught?
I feed all my forest type torts fresh water shrimp (basically small crayfish) when available in store. I feel that in natural habitat they eat terristial crusteans when found (i.e. hermit crabs).
I have fed shrimp to my redfoots in the past and when I did I boiled it for about 3 minutes and waited unitl it had cooled down. I personally have not fed shrimp to my RFs in a while due to the fact that I live on the gulf coast and I am paranoid about the oil; but that is my personal decision. Ultimately its your call, and if you do decide to feed shrimp to your tortoises make it a rare treat as it will cause kidney and brain damage if consumed in large amounts and eaten frequently.
Most shellfish is high in cholesterol and phosphorous and low in calcium and not a source of vitamin D, but it is high in proteins and Omega-3 fatty acids.
Personally, I think that you get more bang for your buck by using salmon, mackerel, or tuna which has most of the same nutrients, but adds good vitamin D3 to boot.
But, if you have some shrimp laying around, properly 'safed', why not once in a blue moon?