Different mdbox_rotate_size for primary and alternate storage

Hello. In order to speed up backups of very very old messages I would like to set two different limits for mdbox_rotate_size. Like, 50M for primary storage and 100M or larger for alternate storage.

There is no mention in docs or such a possibility, so I assume it is not possible. Is that correct?

While I am at it, is it possible to configure primary storage as maildir (sturdy indexes) and altstorage as mdbox (more delicate indexes)?

Thanks, Paolo