I have a stone floor, which I wish to shatter parts of, and then move those parts. Would using a group modifier, as it is higher than part, allow this, or would it need complexity, or two separate spells? (assuming all normal requisites are met, such as perdo and rego)
Personally, I'd say two separate spells for the breaking and the moving. As for Group/Part... Huh. Interesting question. I'd say Part on the spell that breaks part of the floor, then either Ind or Group on the broken part(s), depending on whether you shattered part of it into pieces (Group) or cleanly cut it out as one piece (Ind).
Agree coverage of using T:Group is enough to allow for the T: Part.
Also think you can use T:Group and include the breaking effect as a sub-spell within by using requisites, so add +1 Mag. Totally agree that it could be creates as two separate spells, but it seems reasonable that the first shattering is a momentary aspect.
I think there is precedence to support this approach - The Crystal Dart creates and flings a terram object (MuTe/Re, Ars p154), so no reason that your effect could not do something similar.
I believe one spell to break and move part is enough.
Rego with Part as target will allow you to strip a part of something: The Earth's Carbuncle does that. The Muto requisit turn the ground into jagged stone, the Rego component moves it into an explosive form. No Perdo is needed.
Regarding Part vs Group, you probably can also go with Part +1: instead of one part, you have 10 parts or a larger parts (which by definition can be as big as Individual - p113 "Part can be up to the size of the base individual"). It is a bit of strech as it seems that Part is always limited to a single component.
By rule, you can increase Part size by adding magnitude, it is not mention that you can handle several Parts. But since we achieve the same difficulty level as Group with Part +1, it seems okay for me.