For softball, 300mm is easily enough or more than enough. I've shot plenty of kids baseball with my SMC-A 70-210mm, and also with the Tamron 70-300mm but never really had to use 300mm much. Remember 300mm in APS-C is 450mm equivalent, so it's quite a lot of reach when you think of it.
As for wildlife, it really depends on what you mean. Tiny birds or that King Heron by the pond
? Depending on what your expectations are, 300mm can be a good start and the 55-300mm PLM is a lens you're likely to always carry with you because of the small size and the sharpness, but later you might want something longer. I'd say starting with the PLM lens is a good idea and it's not like you'll be regret buying it - this lens will be your softball lens, period. Nothing else focuses as fast, even lenses costing much more, and the results will be good.