If I am only using rcv for FS9 is it necessary to download the large file or is the update only to make rcv compatible with FSX?

no

Am I right in thinking that there is no harm in downloading and installing RC4.3, even if only using FS9?I am thinking in terms of later updates, where normally one needs the lastest previous version before downloading the upgrade. I would have thought it would easier for you to supply upgrades or patches when as many as possible are on the same version.

yes you can, will not cause any problems.i doubt there will be any more updates after 4.3except v5, which really isn't an updatejd