"Mousies" is the plural of "mouse", as in "I have these two three-button mousies just in case I spill the grapefruit juice on one". This avoids the awkward situation of calling it the same thing as a plural of the rodent.

"Mouses" is what a mouser does - think Tom and Jerry: Tom is a mouser. He mouses. He mouses Jerry and any other meece (meeces?) that might populate the dwelling.

Coffee, please! Not lucid yet!