1) If a person is standing and praying, and facing him there is a non-Jew holding a symbol of idol-worship (such as a cross) and he reaches the part of the Amidah where he is supposed to bow down, he should not bow down as even though he is intending the bowing down for Hashem, it nevertheless appears as if he is bowing down for idol-worship. (Shulchan Aruch and Mishne Berurah 113)
2) When praying, one may not add to the praises of Hashem more than already in the fixed version which is “The great, strong and awesome G-d” which is the version that our teacher Moshe himself wrote in the Torah and the men of the great assembly fixed into the prayers. However, when making other supplications and entreaties such as people do, he is allowed to add praises of Hashem, but it is nevertheless more correct to use expressions used in the verses of Tehllim which contain many praises of Hashem rather than saying expressions which he makes up from his own imagination. (ibid)
