Question 1

Can this device support access by legacy Win98 clients?

Accepted Answer

It uses SMB, so I don't see why it shouldn't support Windows 98 clients. Remembering Windows 98, I could imagine you might run into NETBIOS issues, which could be resolved by giving the NAS a static IP address and either adding a record to each computer's HOSTS file, or mapping just mapping drive letters to the NAS's IP address instead of its hostname.

By default the Guest account is enabled, which would alleviate any authentication issues you would have to iron out with usernames and passwords, although this should only be done if the device is on a private non-business network, your WiFi is protected, and there is no sensitive information stored on the NAS.

Question 2

Is it compatible with two 6TB HDDs?

Accepted Answer

I haven't used drive that large in it. 
You can look up the model of your hard drive in their compatibility chart. Newegg won't allow me to place links in the responses, but you go google "synology dot com" (you know what I mean) and "compatibility" it should come up.

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