I'm thinking about getting a 5 blade prop to have as a spare and to improve rough water performance on my Z9 250 PRO XS. Will the 5 blade prop help me plane at lower speeds? I am currently running a 26" pitch 3 blade, what pitch is recommended for the 5 blade?

Why would you want a 5 blade?
If its not broken don't fix it

Never had any use for a 5 blade prop on a bassboat. I would make a quick call to Steve and get the right answer to your problem.

http://www.stevescustomprops.com/

a 5 blade prop on a bass boat is a waste of money.

I have a Merc 5-blade on my Merc 200 EFI on my old 19-ft Skeeter. The boat was designed not to use a jack plate, so I didn't get one. The prop does a great job on the hole shot, but is 2-3 mph slower than a 3-blade.

You'll probably pay more than $100 per blade.

I have one and I hate it. It was my spare busted up my 4 blade. I am running a 19ft sprint with 200 johnson.

In the OP's case I would look at a 26 Bravo 1 XS pretty hard.

The high fives work best on under powered ski and pleasure boats. I have put several on Sea Rays and Crownline type boats this year. Those people love them.

I have an older Skeeter Starfire and I run a 6" jackplate and 175 Merc a Turbo-1 23 pitch and get real good performance. Wouldn't have a 5 blade on a bet.

When I got my boat back in 07 there was very little history on the 250 Pro XS/Prop setups as the 250 was just coming out. I had to go through many props to find what worked best. Here is some of my most pronounced findings:
Merc Tempest 26p 3 blade absolutely will not work on my boat. Can not keep from blowing out and has no bow lift.
Merc Trophy II 26p 4 blade is best overall but had to get worked on to get better bow lift at top end. Gives great bow lift control at low speeds in big waves and has really good hole shot. Can even go to 25p and get better hole shot but give up top end. This prop has one problem, I have broke three of them without hitting anything so if you run it have a spare in the boat at all times.
Turbo Lightning TXP 3 blade 26p has great mid to top end lift, is very strong and fast. Is on my boat all the time except when I know I am going to be fighting big waves then I go to the Trophy.

The only use for a 5 blade would be big water up north like on erie!

IMO a 5 blade will be harder on the lower unit than a 4 blade, especially in rough water.

5 blade is really not much use down here. Theres almost never water big enough to warrant that kind of grab.

if your boat has problems getting up on plane the hi=5 will improve it.
Also if you fish skinny water a lot you can get up on plane in 2 foot as long as you don't have to worry about stumps and rocks.

It will cause you to loose 3-5 MPH on the top end. I ran one for 16 years in all kinds of water and except for the loss of speed had no complaints.

Was great in rough water too.

I have never had a 5 blade. I have had a 4 blade and loved it.