I'm not sure if the Pluto you have in mind was pre-signed or if your cousins gathered the signatures themselves.
I personally have not seen any plush toys signed by multiple characters in any of my recent trips to WDW. Nor have I seen signable stuffed toys recently (also known as "autograph hounds") at WDW; I've only seen fuzzy animals that would not hold a signature well.
However, there are MANY opportunities to get character autographs while you are at Walt Disney World. This is a big hobby with many people. There is even an independent website devoted to this pasttime: wdwautographs.com. You can walk up to characters at any greeting line at WDW and ask them for an autograph.
Disney sells autograph books at almost every gift shop on property. But you're not just limited to gathering autographs in books. I have seen or heard about characters signing calendars, t-shirts, baseball caps, photo mattes, posters, frames, and just about anything else you can think of. As recent favorite item that I've seen signed is a blank 9-inch Vinylmation Mickey doll. Go to the website stitchkingdom.com and search for an article called "Vinylmation Mickey: The New $40 Autograph Book". I simply can't get over how cool I think this is. The blank Vinylmations are sold at many of the larger WDW gift shops.
You can read more answers about autographs
HERE,
HERE and
HERE.
Have a wonderful time at Walt Disney World.