That's the nice thing about the MMS APN, it's separate from the general internet data APN. You should theoretically be able to configure one without the other. That said, there's nothing keeping you from restricting mobile data usage on the account and handsets, either. Pick the 100MB data bucket, restrict data usage to around 20MB per line, and either disable all mobile app data access on the phones except MMS, and/or place a monthly hard data usage cap in mobile data settings.