If you can stop the fertilizer dosing for awhile, I think that would help. When I set-up my first cherry red shrimp tank I wanted the plants and moss to grow so I used a well branded fert. My shrimp did well for awhile but I ended up with all kinds of things in that tank which I contributed to the soil type substrate to the fert's. I re-did the tank with normal aquarium safe gravel, java moss, and anubias on driftwood. I stopped the fert's and increased the light out-put but decreased the amount of time the lights were on. This worked for me on the second go around.
Limpets are annoying but they wont hurt the shrimp. I have them and the shrimp are breeding well.
Water changes will only be beneficial so dont hold back but try stopping the dosing for a bit and see how things go. Your grass may suffer but your shrimp may benefit.