However, there’s one campaign manager I noticed who doesn’t allow that, and that’s Hhampuz. Based on my observation, if the opening is for a Sr. Member, he will only hire a Sr. Member, even if a higher-ranked member applies and agrees to a lower rate.
The OP is not talking about accepting accepting giving lower rank slot to higher ranked members. He is only talking about the reason higher member pay is more than lower members.
There’s no general rule for signature campaigns in the forum. In fact, you won’t even find anything about signature campaigns in the official forum rules.
You're right, there are no general rules about managing a signature campaign, but the forum has guidelines for signature campaigns which can be found in the pinned topics in the Services section or you can check it here:
Signature Campaign Guidelines (read this before starting or joining a campaign).
But I don't know if this is still being implemented because since that guideline was posted, only a few have actually been punished or banned for not following it. I know there are some who violate it, but like others, I just let it be to avoid any issues with the said violating account. As for me, as much as possible, I avoid arguing unless I know that I am right.
This is not about forum having a general rules about campaigns but about campaign managers. There are no general rules about it but on almost all campaigns, lower ranks are paid low than higher ranks.