I don't think you should give your daughter a dummy to stop her sucking her thumb - you'll probably find she ends up sucking on the dummy to sleep and sucking on her thumb when she's awake, rather than doing one or the other - or rejecting the dummy so that she can suck her thumb, anyway.
As for speech impediments, I don't think this is normally an issue unless you let your child have a dummy almost all the time or until they are well past the normal age for having dummies (I do know one child who is now almost 3 and still has a dummy in his mouth a lot of the time, and no-one can understand what he's trying to say - largely because he tries to talk through the b*dy dummy rather than take it out of his mouth, although he sounds rather odd when it's not in his mouth, too, it has to be said).
And as for whether you try a dummy for helping your baby sleep, I think there's no right or wrong. I didn't like them, the only occasions I ever tried them on my children they gagged and spat them out, and I breastfed, so found the idea that I might have to sterilise them rather offputting, since I wasn't sterilising anything else! Oh, and I didn't like the idea of having yet another thing that they would eventually have to be stopped from using, having got them used to using them as a comforter. But if you're getting a bit fed up with walking around with your child to help her sleep, then why not try a dummy for a bit, and if it doesn't make much difference then stop using it, and if it does, then you've done the right thing for you.