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Siman 112 . Not interrupting in the first three or final three blessings of the Amidah

112:1. (1) One should not request his personal needs in the first three or final three blessings [of the Amidah]; communal needs, however, (2) are permitted.

MB 1: One should not request - The first three blessings are compared to a servant arranging his master's praises; the middle blessings to a servant requesting sustenance; the final three blessings to one who has received sustenance and then departs. Since the first three blessings are designated for praise [of the master], it is not proper to beseech personal requests, and the same reasoning is applied to the final three blessings.

MB 2: Are permitted - Since communal requests is also part of [His] praise, as [the fact that] the community relies upon the Superior honors [and gives significance to] the Superior.

112:2. One should not recite insertions (lit. P'yutim) or poetic liturgy (lit. K'Rovetz - some say that the word K'Rovetz is an acronym Kol Rinah V'Yeshua B'Ohalei T'Zeikim [a voice of praise and salvation in the tents of the righteous], one such insertion) [in the first or last three blessings]. {Rama: (3) Some permit [insertions] since they are for communal requests, and so is our universal custom, to (4) recite them.}

MB 3: Some permit - The responsa of the Ridbaz (Siman 532) goes through great lengths [to justify them] and permits them, see there. And see there where he cautions strongly against changing our custom. See also the Bach Siman 68 and the responsa of the Chovas Yair #238, who says that it is proper to recite Insertions and not to change our custom.

MB 4: Recite them - See the Pri Chodosh who warns that it is important not to delay past the proper time for Shema and the Amidah because of insertions, and also not to fast half the day by delaying the [Shabbos or Yom Tov] meal past noon because of them. One who is praying with a Minyan and sees that the recitation of insertions will cause a delay of Shema past the proper time should not wait for them; rather, he should recite the blessings preceding Shema and continue to say the Shema [in its proper time] until the end, and then wait for the Minyan to catch up. While he is waiting, he should remain quiet and not talk (even in Torah learning), until the Minyan catches up with him, and then should continue his prayers with the Minyan.