scotts33 To the OP what I would do is build a test iron and try it before building all the irons
Bingo. Build a demo iron. The shafts are inexpensive.
I have been personally using the FST 115 stepped shaft for years. I have never tried the Pro series but have read where they play softer than the stepped FST shafts. I find the FST shafts to be better feeling and "smoother" than True Temper or Apollo. I started trimming my FST shafts to a R flex and now at 68 shafts trimmed to a senior flex per the trim chart fits me well.
Now, regarding single length. I think a big mistake many make with SL is trip trimming the shafts. Many decide they want to play the clubs at a 6, 7 or 8 iron length. So, if 6 iron length they tip trim the shafts for a 6 iron. Nooooo. Tip trim the shafts based on head weight. If your heads are what a non SL 8 iron is then trip trim them for an 8 iron. Then, if you want to play them at a 6 iron length butt trim them to your 6 iron length. I have played about 5 sets of SL irons. I have found that an 8 iron weight and length works best for me. I currently play Wishon Sterling SL's at 276g head weight and have the shafts tip trimmed for an 8 iron. I also play them at 38" including the wedges.