So, what happens if I don't specify a quota for any mailboxes under a given client, but the client template that's applied specifies 10000MB?
You cant do that as a client. A administrator can override quotas of course, but a client cant. Try it out yourself, set a limit of 1000MB for the client in the client limits, then login as this client and try to create a 500mb plus a 600mb mailbox, you will get a error like this:
"The max space for mailboxes is reached. The max. available space in MB is: 1000"
when you try to create the second mailbox. if the second mailbox is smaller then 500mb so it fits in the client quota, you will be able to create it.
Same happens when you try to create a unlimited mailbox as this client as unlimited exceeds the allowed quota size.