Scaling and Recommended items?

One of the few times playing against Nautilis today. Fiora was top and I was xin jungle. He got two kills on her before i went to gank. We still all had starting items, when we fought we took him down 1/3 health but both of us ended at half he went on to destroy us. Another game against an amumu he had all armor no ap but still dished out good damage. Should I stick with game recommended items more? Do those champions and others scale with armor even when not specified? Sorry im a noob