Yea I would be reluctant to put flag fish in with shrimp. They do a terrific job on algae including BBA. They do leave snails alone. However, shrimp move, and flags seem to trigger on movement instinctively. They're very much like a wild bass or perch. I had 6 juveniles in a 15 gallon tank with one CRS female who managed to survive just fine with them until I changed the sponge filter for an oversized (for this tank) Aquaclear. I'm not sure what did her in - the massive water movement or an opportunity the fish took advantage of.
Also, this old saw about lack of this or not stable enough that...you know. I can show you a spraybar that distributes the CO2 saturated water out of my reactor with BBA just sloughing off of it. Here is a good article about balancing, and some things to consider. There's another forum where there is active discussion about this topic, and the back and forth is well, illuminating to say the least.