We had this topic regarding requirements on centos ish systems quite some times in other directions already.
yum does not support optional or alternative packages. CentOS 8’s yum does in specific versions, however, as this is not always the case we decided to not go for alternatives for yum based installs at all.
You’ll either have to break dependencies (which is on your own danger especially in regard of possible dependency changes) or install pgsql and disable it. I don’t see the risk installing a pgsql next to mysql. Zammad has its configuration file for the database. As long as you don’t tell it to use postgres there, you’ll be fine,.