Can I just go with an FHA loan even though I qualify for Conventional with 20% down?
Yes, as the borrower you can choice the mortgage type and product you want.
can I use conventional financing to buy a 2 unit property?
Can I put 5% down payment and buy a multi-family house?
Yes, you can put 5% down or less and buy a multi-family house using specific loan programs like FHA loans (3.5% down), conventional loans (5% down), or VA loans (0% down for eligible veterans).