There have been debates about this. Just make sure that you practice with a light ball after then finish with a regular softball. and make sure that your form doesn't change when you use a weighted ball

http://www.softballperformance.com/softball-drills/softball-conditioning-drills-to-work-on-stamina/

I long toss with a weighted ball. All that happens is that the weighted ball makes me push more with legs and helps create more power. It just comes naturally to me.

I've been using a weighted ball for a few weeks now. I feel like my speed has picked up. At try outs I was clocked at 39 mph. I will let you know in a few weeks if it goes up.