[Avodah] "Ancient" Minhagim

[Harvey Benton]

RAMiller wrote (not exact quote) that we have no principles to guide us other than each individuals responsibility.... to find Truth (re adoption of new minhagim).            HB:  What about injunctions such as Lo Taturu, not to go against the words of the Sages, and Bal Tosef?  Has nothing been codified in this regard?                     Further, why have post-AKHagedola prayers been allowed into the siddur (some with the name of Hashem)?  Ex. include Vlamalshinim, Lecha Dodi, prayer for the State, LeShem Yichuds, etc, ?  HB