Ideally, you should avoid to mix styles with skinparam and try to only use styles, especially if you write new diagrams.
Now that's the theory.
In practice, we don't want to break existing diagrams and we want to allow users to softly migrate from skinparam to style. So at some time, you could have a mix of styles and skinparam. We have made our best so that those mix will be working.
So to answer directly your question, you can add new styles feature in C4-PlantUML even if the original parts still uses skinparam. There might be issues (because testing those mix is difficult), but if you find any, you can post messages here and we will hopefully fix them.