Up here #3 is used for a 100 amp panel and 2/0 is used for 200. I can’t remember what a 150 requires but it would be somewhere between those two. Seems to me the general rule for using aluminum triplex was to go one size larger. I can’t ever remember seeing a breaker under 15. Maybe back when panels used fuses I might have seen less tha 15.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.