The reason I decided to not use a smaller tank for the retract system is that I feel the one suggested for the kit is way too small for the application. You should have at least six full cycles of air supply for your gear as insurance. I personally like to retract my gear and extend them in flight to watch their operation, especially with sequencing doors. Two or three cycles of air reserve is cutting it too close especially if you develop an air leak in flight.
A sufficient substitute for this medium air tank would be to either use the Robart Large air tank (Cat #192) or two of the small air tanks in tandem (Cat #172)