A landlord can accept an In Principle Approval Letter. However, you must provide your valid Work Permit or Employment Pass as soon as it is processed.
A landlord is required to check original NRIC or FINS of the tenants and occupiers for forgery and make copies, check photographs on NRICs or FINS against the actual persons to confirm identity and verify the validity of the passes with MOM database and/or ICA database. As an added due diligence, they may also ask for letter of employment from employer. This is to deter illegal immigrants or vice activities. This is required by the Council of Estate Agencies for compliance with the Immigration and Women's Charter.
You will find our rental guide here useful: https://www.propertyguru.com.sg/property-guides/singapore-expat-renting-guide-36336
Hope this answers your question.