Why Smaller?, If anything at all a bigger cap... 100 - 470uF would be better and probably as big as is required. anything larger would serve little purpose. If you have the board wired Exactly as shown (A Star topology) then the only other thing I might suggest would be 1N4004 diodes across the motors cathode to the positive side, anode to ground. The star wiring method insures that all currents return INDIVIDUALLY to a common point so that ground current isn't "Shared" along with the noise due to "Daisy Chained" grounding. Because I am rather conservative (from painful experience) I develop all my circuitry on breadboards, a practice I've used for 40 years... All of what I design works right when I make a PCB from my final breadboard design. I use breadboards for design not so much because they are easier but because they are about the worst possible device to use for low noise high current loads... If I can make it work there I know (Again from painful experience) it will work about 99.5% of the time. Murphy is always present... And he was an Optimist.
Bob