> "Many were incorrectly labelled as SI_xxxx" 

Well, that's what the signs say. So they shouldn't be deleted since they do exist and do have this exact wording printed on them.

railway:signal:*:form=sign indicates that this is a static sign rather than a light. 

If some yellow signs have a different purpose, you can indicate that with railway:signal:*:function - see https://osm.wiki/OpenRailwayMap/Tagging


> "The resources are public. [...] I'm not sure why we couldn't use them."

Unlike wikipedia, we can't just use copyrighted information, we need explicit permission. See http://osm.wiki/Copyright and https://osm.wiki/Contributors#Australia for a list of australian sources. If you want to use this information, you would need to email the company to get explicit consent