should I have all my devices on the same iCloud?
I just realized my iPad was originally activated with a different iCloud account than my iphone and iMac. How can I merge them or would it not be a good idea. I have info on 2 different clouds?
You cannot merge them per se. If they are two different iCloud accounts then they are attached to 2 different Apple IDs. Any purchases you have done will be tied to the ID that you purchased them under, so if you have made purchases with both Apple IDs then you cannot "merge" those.
However, when you sign out of iCloud on a device, you have the option to keep your Contacts. Those can be merged with whatever iCloud ID you then sign on.
Photo Stream is specific to the iCloud account it is turned on under. So, again, that would not be able to be "merged".
On the other hand, having the same iCloud account on all devices will ensure that they all get the same Messages, Mail (if you are using iCloud Mail), Notes, Photos, etc.
Cheers,
GB