Bogul is running a great service by providing fair associate rates to user(s) who apply via his guild forms. If there's a hard coded limit to the number of user(s) one can add, this effectively hinders any plot owners from providing discounts, especially when, as said above, the only way to bypass this is via alliance and/or guild tagging.
People will not leave their main guild or alliance simply for associate rates on city buildings.
The 60 entry limit seems like a random hard-coded integer somewhere.
I would suggest increasing city plot buildings to ~200 or 300 to match guild limits.