A Few Questions About Clicker Training A Dog?

We’re adopting a two year old Retriever/Greyhound from our local humane society. I am not certain as to the level of training she has and want to try out clicker training with her.
I can’t seem to find definitive answers to the following:
1. When you click and treat, do you also praise? Should I be saying, “Good girl!” while I am giving her the treat?
2. Do you correct at all with clicker training? I realize it is positive reinforcement, but does that mean that one does not correct? No saying “NO!” or anything like that?
Thanks to all.

Clicker Training A Dog?

I have a 1 year old italian greyhound/chihuahua mix. He knows many tricks such as sit, lay down, shake, high five, play dead, roll over. The thing is, it sometimes takes him a while to do these tricks. I think Clicker training would be a good way to teach him to do these tricks better so as soon as I say the command, he does it. Sometimes now, I have to pat the floor for him to lie down, and it gets annoying. I was wondering if clicker training will still work even though he knows some tricks already. Will I confuse him if I “back track”?
Also, what treats are the best for clicker training, because I know you have to use a lot but I don’t want him getting overweight.
Are clickers sold at Pet Supermarket?
Any other info? thanks!