We were discussing your quota problem with Roundcube (not Virtualmin) here: Disk quota monitoring on Virtualmin - #8 by almatecstaff
There are no significant differences in Disk Quota support between Pro and GPL. Pro is focused on features related to large-scale hosting and features that help hosts make money with Virtualmin. Quotas are not among those (there are some additional features for doing bulk updates of settings and such in Pro that can apply to quotas, but they are not relevant to the problem you’re experiencing).