One tape drives are replaced make sure drives are visible from NW server operating system as well as from SN operating system using inquire command. Then reconfigure the library.

Is your data pertinent to your business? Have you considered upgrading to networker 8.2?

Is there a reason you are staying with tape instead of other more modern means of storage?

Semper fidelis et paratus, /ALE

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Keep in mind that you need a jukebox license which covers the whole available number of slots for each jukebox .

If you replace the jukebox right away with the same hardware then this is no issue. However, if you want to run the new one (temporaily), you need at least a temp. enabler. Such are listed in the NW 8.2 Licensing Guide.

The number of drives is indirectly determined by the server and storage node editions. 9 drives will not be any problem.

Unfortunately, you do not state the exact nature of your hardware problems.

The major issue I see is the mix of different tape drive types within the same jukebox. AFAIK such is officially not supported.

Why? - because NW cannot guarantee that the correct drive type will be used.

You can limit that to a certain extent if you specify certain drives to a pool. However, this will 'only' result in mount restrictions for the media. But keep in mind that there are other processes where a tape must not necessarily be mounted (like label & scan jobs).

Let's assume that you have Auto Media Management enabled. If NW needs a new tape, it could end up loading an LTO5 media in an LTO4 drive. As a good backup software, it will first try to read an existing label to verify that a valid backup tape will not be overwritten. At this stage trouble will be obvious.