To add to Chris’ reply:
If yes, do we still have to use the 6 motion parameters (because this nifti file is already motion corrected using FSL and ANTs) ?
The short answer is: Yes. Those parameters are for denoising, not for performing motion correction.
Motion correction aligns the EPI volumes so that the same voxel refers to the same location in the brain across all volumes, and those parameters are the output of that process. However, there may be noise components that correlate with motion, and you can thus use those parameters to correct for that. (See, e.g., Friston, et al. 1996.)