As subman stated, if you are planning on running both phosphate remover and carbon, you will need 2 reactors
You can run them in series with 1 pump, but it's easier to run them parallel with 2 separate pumps
As far as lowering your pH goes, it may happen, and you need to keep an eye on your Alk and Ca during this time to see how your tank settles into it's "balance"
Don't go all crazy adding additives until you've read lots about it and how to keep it stable
Want some reading to learn what affects pH ?
Here, Myka explains some of the basics, etc
This article further explains it, with some charts that make easy to visually understand things