There not are fixed levels for nitrates and phosphates so the standar range is 10-20 ppm Nitrates and 0.5-2 ppm Phosphates. Dont worry rising NO3 to 10 or 20 ppm fast, Its not is toxic at that levels.
Shrimps in 55G is kind difficult, if you are talking about cherry shrimps or similar size you need a lot to see them. Still almost all fishes eats them so I dont know about your fish but I saw people with tetras and no problems and other people with tetras that eats shrimps fast, so I think it is more a behavior thing, than specie. I have guppies with shrimps they dont eats adults but I think that baby shrimps are easy food. The tanks with only shrimps have a lot more than tank with shrimps and guppies.