I think you want to capture Lead Source which should be in SFDC by default. If a lead comes in by Phone or an email box, I train everyone to use'
Lead Source = Inbound Call (They Called Us)
Lead Source = Inbound Email (They Emailed Us)
SFDC Type = Lead, Contact, User which is a separate field with different implications.
Yes, if you set a field to be a required field to create a Lead, then yes, SFDC will reject Marketo's attempt to insert the record if that lead is missing that data.