With Small World, yes you can. I just booked with them yesterday for a cruise, paid the deposit and then set up for monthly payments to be applied to my CC. As the agent explained, they just make the payments for you to Disney..charges on the CC will show Disney..the money never actually goes through Small World's hands. They had the exact same price as Disney for the cruise, the only benefit was more onboard credit.
I was on an Adventures by Disney Backstage Magic trip with Sue, the owner, a few years ago, and she's a nice gal. My agent for the cruise is Lynne.