When saying the Harachaman in birchat hamazon for the host of the meal are there any rules? May I bless whomever I like? I.e. If I am in my own home or someone else’s can I also say "avi mori", also should I say "et zari" before I have children?

In the Harachaman prayer one may bless their wife/husband, parents, children and hosts. There is no prohibtion to add others in but it is recomended to stick with the widespread custom. You can bless your wife/husband,parents and children in any place you are eating. If you are eating at someones house you will simply add in for the Ba'al HaBayit as well. The custom is to say the blessing of the guest (Birkat Ha'Oreich) for the owner of the household. (Yhi Ratzon ..) One who does not have children may not say the 'et zari' prayer for their future children. Either way, may you be blessed to begin saying this prayer very soon! Rabbi Binyamin Bemberger