6-units should be doable, I'm guessing you are looking more along the DSCR route on it? Do you already have it under contract and have the financials behind it?
How many of the 6 units are occupied? If the NOI is tight, you might want to go with a DSCR program that goes off of Rent/PITIA. If the loan amount is high enough and the NOI is there, you can go for full doc or a program with more stringent UW model for better rates.