calibre doesn't have a "number of slots"
certain formats are builtin -- which means they have input/output conversion plugins -- but you can add literally any filetype whatsoever, even made-up ones.
I don't know about optimizing for different devices -- seems mostly useful for PDF, which I don't use.
And it's a lousy format anyway
ideally the books would be OCR'ed and proofread then turned into reflowable EPUBs.
You could create multiple book records, i.e. "Some book (%device% edition)"