67 year old experimenting with single length.  Read that cutting shafts makes it stiffer.  Thus if using regular flex shaft’s now do I need to use senior flex sl to get regular flex.

Answered question

Sounds like your trying to use your existing shafts to get to single length and that won’t work.
You will need a new set of shafts of whatever flex you require and trim them all to 7 iron spec (tip and butt) or 8 iron. Also all the heads need to be the same weight.