The hull of the Erie was basically identical to the Treasury Class Cutters ie one of the most cost effective class of vessels ever paid for by a nation's taxpayersJust to avoid problems, they could state that monitors are ships with one twin turret with 10" or larger guns, under 12,000 tons, and under 18 knots in speed. All signatories can build up to 100,000 tons of these. Anything bigger, or faster, or with more guns, is a capital ship and counts against capital ship tonnage. This should avoid attempts at cruiser-killer monitors and other "interesting" ideas.
It will be interesting to see what this treaty comes up with as unrestricted military vessels. IIRC the OTL WNT stated that there were no restrictions on in the total tonnage of such vessels as long as they were under 2,000 tons, had no more than 4 x 6" guns, and had no torpedo tubes. I think this produced the Erie class gunboats.