No, the 18 deg North facing roof is not a big problem (unless you live in Tassie or somewhere very far south and use a lot of energy in winter?)
In summer the lower pitch will actually slightly boost total power but in winter when the sun is lower on the horizon it will reduce slightly compared to 30 deg pitch but only by 15% or so.
Depends what latitude you reside in the world. You can look up your ideal azimuth angle on-line for different times of the year. But in reality you will stick with your roof pitch angles. If there is concern, you could add an extra panel or two to compensate.
I hope you are placing the 18deg North facing on different MPPT ( or Group )
Are all 28 panels on the same inverters?
Are there more then 1 inverters at this site?
and what is the wattage of each panels?
Regards Esmail