The Project X will be slightly lighter, slightly lower bend point and will likely spin more. Just grab a Vokey wedge shaft pull (Dynamic Gold S200) on ebay for $20 and throw it in there so you have the same across the board.

True Temper Dynamic Gold S200 is what the WedgeWorks uses and calls their standard “wedge flex”. You can research the differences between that shaft and the Project X. I personally try to keep all my wedges with the same shaft.

It's a better question to ask a professional fitter. But, if you can test the 54° with your existing wedges and check weight, length, and launch performances. I prefer to have my irons and wedges have the same shaft. Good luck.

For consistency, I would get the same shaft I have in my other wedges. I always get my wedges thru my local fitter and this is his recommendation. Hopefully the 54*'s lie and loft match your irons and other wedges.

Wedge flex and stiff flex are basically the same. Just make sure the project X is the same weight, or heavier, than your other wedge shafts and you should be fine

Wedge flex would be specific to the shaft manufacturer so the actual flex will be different between manufacturers. Some people advise using the same shaft as the irons and perhaps a little softer by using the .
same shaft as your set but use the 8 or 9 iron shaft in the wedges. It would only a guess (likely a poor one) to suggest how the PX LZ 6.0 would compare to your "wedge flex" shafts.

I agree with the same flex to the point of putting my iron shaft into my Vokey wedges that I got instead of the PW that is the same as the set