It's ordinarily a good idea to await the family report before writing your next affidavit because if the family report comes back with any problems, you can demonstrate with evidence how you are able, or have already, addressed these problems.
I would file an affidavit supplementary to your initiating application before the interim hearing though, rather than amending your initiating application, because the judge will likely want to see what progress has been made toward fixing the issues. You can file an amended initiating application if you are seeking different orders to those sought initially, though obviously you would file a new affidavit with it, rather than changing the previous one.
I have always viewed affidavits for interim proceedings to be a 'progress update' showing how you've started resolving problems. For example, if there's trouble with communication between you and your ex, and you've enroled in a post-separation parenting course to help improve communication, you would provide this in an affidavit before the interim hearing.
As always though, please get legal advice. Legal Aid offer free consultations and they are very helpful.