Yes that config looks OK to me. I don't think you need to have multiple backup to disk folders for the different jobs, but since you've only got two jobs, and it won't make a difference to the space used I'd probably go the same route as you have, just so it's easier to keep track of what's going where.
However, firstly you're getting your terms confused. A brick level backup is exactly what you're proposing above, it relates to the fact that you're backing up each individual item (hence the brick metaphor), rather than the Exchange Information Store as a whole.
Secondly, I really wouldn't recommend only doing a brick level backup of your data. If you have to choose between a proper Information Store backup or a mailbox backup due to space or time, you should go for the Information Store option as that's the only one of any use to use if you have to do a full disaster recovery type restore.
Additionally, if you're using Exchange 2003, if you ever need to recover a single mailbox or mailbox item you can use the new Restore Storage group to recover the data without effecting your existing data.
Finally, if you're only doing mailbox backups, with no backups of your Information Store, as far as Exchange is concerned it will never be backed up (mailbox backups are not considered a backup by Exchange), therefore the logs will never be flushed, so they'll just keep growing.