Orach Chaim 603 says that a person should avoid eating Pas (bread/baked goods) made by Kuthim (or non-Jews, Mishnah Berurah, ad loc.) during the Aseres Yemei Teshuva. Does this include cereals made from the 5 grains? (Cheerios, Shredded wheat, Honey Bunches of Oats, Fiber one, etc.) A sourced answer with an explanation would be appreciated.
This would depend on whether or not someone would make hamotzi if eating a meal's worth of breakfast cereal. To paraphrase from http://www.halachipedia.com/index.php?title=Pat_Akum:
Breakfast cereals which are Mezonos would be subject to Pas Akum only if they have tzuras hapas, because otherwise one would never make Hamotzi on them (Rama O.C. 168:13, Shulchan Aruch O.C. 168:15, Mishnah Berurah 168:3). Rav Scheinberg zt"l (as quoted in Vezos Ha’beracha page 192) thinks that cereals such as Cheerios should be considered to have tzuras hapas, but the OU's policy, as determined by R. Schachter, R. Belsky, and R. Genack (in OU Document A-94) is that breakfast cereals such as Cheerios are permitted to be eaten when made by a non-Jew, because they lack tzuras hapas and are also not fit for a king's table.