I'm trying to add a plated via however it always adds a cross around the via.
I set the via to be part of ground but I don't want to touch the other ground vias.
I understand that I can add a rule for that, also to define the component there but free floating vias do not show up in that list?
First a via is plated. An unplated via is called a hole
I assume by adding a cross around the via you mean a thermal relief.
Thermal relief generation is controlled by "polygon connect style" and "plane connect style" rules.
IsVia will match vias in the rule.
If you want vias on the same net some with and some without thermal reliefs you have to identify what you want somehow. Using a particular via size for those you want directly connected might be the easiest thing to match in a rule.