In respect of whether to use fine or medium grade clay, only a 'test spot' will tell.
The golden rule, as it were, is to ALWAYS use the least aggressive 'tool/product' to get the job done.
So if it were me, I would start with the fine grade and see the sort of results you are getting and how long it is taking you. If the results are acceptable, you can proceed with the fine grade clay, which should be less prone to marring the paint.
At this point, I would inspect any possible marring and ensure I could remove this, before continuing on the whole car.
If however you are getting nowhere slowly, stepping up to the heavy duty grade may make the claying much quicker, but at the expense of an increased likelihood for marring. Again, check you can remove the marring adequately before doing the whole car.
Do you intend polishing after decontaminating?