penis pumps

Iv been thinking bout getting the Tracey Cox Penis pump.
The thing is i have never had a pump before so dont know how suited ill be for it.
Should i use a cock ring and if so do i put it on before or after pumping?
I also understand that pumps increase ur girth then length, im told my penis is very thick already, do u think being even thicker would be a problem?
Curious to hear other ladies thoughts on this.
Thanks
Pete