What are the different sources for covering one’s head in a Talit during Tefillah? What are the different Minhagim? If one is a Yekke - but does not usually wear a hat - should they cover their head with the Talit or should they not?

The Shulchan Aruch 8, 2 writes that one can cover the head with the Talit. The Mishna Brura, 91, 6 writes that at least during the Amidah the head should be covered and ibid 7 adds during the Shma too. There are many customs for covering the head – some cover their head throughout the whole Tfilah, some have the custom that only married men cover. I am not familiar with the Yekkeshe custom but all customs are acceptable. (see more in Ben Ish Chai Bereshit 5, Kaf HaChaim 8)