Is it a bad idea to have a shared Apple ID for purchases that also is used for iCloud? There is lots of advice that suggests using a shared Apple ID for purchasing things (so that all devices using that Apple ID for purchasing can access those purchases) and to then use a separate Apple ID for iCloud for each "user". 

This makes perfect sense to me, but what should be done if there is currently 1 Apple ID (my account) that is used for all the purchases and also is used for my iCloud, and I want to add another user to the mix?

What problems will there be if my Apple ID is used for the purchases on all devices and also used for my iCloud account for my user, while a different Apple ID is used for iCloud for a different user?

Note: The places this would apply to would be my iPhone, another iPhone, and 2 user accounts on an iMac (one for me and another for the other user)
In your first question you ask:


  what should be done if there is currently 1 Apple ID (my account) that is used for all the purchases and also is used for my iCloud, and I want to add another user to the mix?


To enable you to share purchases the new user will use your Apple ID for the App Store and iTunes. This use would then create a new Apple ID or use an existing one for the iCloud portion. They would assign this Apple ID to iCloud by clicking Settings -> iCloud -> Account. 



Secondly you ask:


  What problems will there be if my Apple ID is used for the purchases on all devices and also used for my iCloud account for my user, while a different Apple ID is used for iCloud for a different user?


This is the solution I noted above, and the preferred method. By separating iCloud accounts for each user you'll have less devices competing for the 5 GB of free space allotted by default.



In summary:


All devices use Apple ID for Store (Settings -> Store -> Apple ID)
Your devices would use your Apple ID for iCloud (Settings -> iCloud -> Account)
Other user's devices would use a unique Apple ID for iCloud (Settings -> iCloud -> Account)
There are no drawbacks to sharing an Apple ID and using individual accounts for iCloud.