Yes Notarized RnR letter would work. If possible, try to get the letters from atleast few companies if not all.
You need to get separate letter for each company wherein a senior from that company needs to certify the RnR in the document.
For format and more details refer below ACS document.
https://www.acs.org.au/content/dam/...t Guidelines for Applicants - August 2017.pdf