You shouldn't add any more fish until you get the ammonia to zero and the nitrites to zero after it.
Also, if your guppies are gasping at the surface, and the surface is smooth, they probably need more oxygen. Set up your filter to stir up the surface or get a bubbler going.
Plants would help both the oxygen and the ammonia.
Once that's done, 5 to 7 of one of the Botia species would be a trip (but would eventually eat your shrimp) -
Yoyo Loach (Botia almohae),
Angelicus Loach (B. kubotai), Queen/Bengal Loach (B. dario), Striped/
Zebra Loach (B. striata). Siamese Algae Eaters, true or false both work - I have 3 in my 75, but 5 would be better. (Make sure you don't get Chinese or Goldens).