Is there a custom a cohen should not marry a widow (apart from another cohen’s widow) in the first place? Are there any communities where it is not customary for a cohen to marry a widow? Why can’t a Cohen Gadol marry a widow? Perhaps nowadays every cohen should prepare himself to become the Cohen Gadol and thus shouldn’t marry a widow in the first place?

I have not heard of a custom to not marry a widow lchatchila (in the first place). It is permitted for him to marry her and should marry her if she is the right one for him. I have not heard of any communities where a Cohen is not allowed to marry a widow. The Cohen Gadol is forbidden to marry a widow because Hashem commanded so in the Torah. On our level it is possible to understand that such a person is on a special elevated holy level and therefore special laws are applicable to him. There is no need to not marry a widow just incase he turns out to be Cohen Gadol. The general principle is that he must marry the women who is best for him and if it is a widow then that is perfect. (unless ofcourse she had relations with a pesul as I have mentioned before). Rabbi Binyamin Bemberger