Are you planning on scaling-out your deployment - if not, then I see no reason not to host everything for SSRS on the same server. It has to be licensed the same anyways so the only disadvantage would be system resources - which are quite minimal for the actual databases.
If you are planning on a scale-out deployment, then separating the databases from the web services makes sense, as that way each reporting services system only has those services available.
As far as I can tell - the version of the database engine won't be a problem, but the Edition will be...for SSRS Enterprise the databases will need to be installed on an Enterprise instance of the database engine.
Problems are opportunities brilliantly disguised as insurmountable obstacles.
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