Hi @Matteo_1987
this is a typical data exchange issue.
You need the highest level of data when import.
the choice whether import as part or assembly is yours: what will happen to the Creo imported model after the import?
Will it be included in another assembly? will you add geometry to it without changing the initial step geometry?
The solution to get the most flexibility is to roll back one step (no pun intended) before the model was a STEP file.
When possible get the data as native, see if it is compatible with Creo import capabilities (find article https://www.ptc.com/en/support/article?n=CS134024) then this will enable you to jump over accuracy issue, edge artefacts, gaps, surface tir and so on.
Finally remember you can use import data doctor to repair imported data whether from STEP or from native file formats.
