Can I install just the stomp/reverb/delay etc presets without installing the rigs:)
No, the import of factory rigs will import all factory content (rigs and presets), which is currently not in your Browse pool. However the factory presets should be there anyhow, if you didn't manually delete any. Enclosed in the firmware download package is a reference list of all factory presets for Stomp/Effects/Delay/Reverb. So you can check if anything is missing.
There should be 123 Stomp/Effect presets, 13 Delay presets, 14 Reverb presets, 7 Amp presets, 12 Cabinet presets, and 11 Stack presets .... plus those, that you might have created yourself.