I would chime in on the side of those who recommend a cool water fish or at least one that will survive cooler water than the tialpia will. Especially if you are going to be cooling the water down for the shrimp (got me confused on that one since I was thinking the giant prawns and they need even warmer water than the tilapia do.) But anyway, warming the water for the fish then cooling it back down for the shrimp then warming it back up for the fish will be very energy in-efficient. So to save a little on heating/cooling costs I would definitely try for a more stable temperature and go with animals that will agree about what temperatures are good.

I've actually found that here in Central FL, even in a minimal greenhouse (no heating) I can grow out channel catfish, bigger/faster than I can grow out tilapia seeing as the catfish will keep eating/growing year round while the tilapia quit for a few months of the year. I expect that without heating to optimum temperatures, bluegill will grow probably almost as fast as tilapia and with heating, they will probably give mixed gender tilapia a run for their money on growth rates.

As to the digging a fish tank into the ground. I would point out that the ground isn't really a good insulator but more of a thermal mass. If the natural ground temperatures are the same as the temperature you want for your water, then digging the tank into the ground will be beneficial. Like here in FL our ground temperature average is pretty perfect for warm water fish. However, if your year round average ground temperature is more like 60 F, then digging the tank into the ground will not be helpful when trying to keep the water at 80 F for tilapia because the ground will be trying to bring the tank temp down to 60 while you are trying to heat it. The earth isn't an insulator, it's a thermal mass and therefor beneficial if you want the temperature it already averages. I would recommend insulating around the tank or perhaps better yet, insulated a large mass under the greenhouse for use as thermal mass and temperature stability for the greenhouse through winter. Then again if you switch to trout, digging the tank into the ground would be a good thing.