Glad to be of help. The lifters must match the cam.
First off, the lobes on a flat tappet cam are cut on a very small angle to rotate the lifters. Roller lifter cam lobes are cut flat, so the roller in the lifter will roll. Second, on the flat cams, the ramps are cut different for solids than hydraulics...ramp approach angles...
The lobes on a flat tappet cam are angled front to rear and rear to front in order to spin the lifters in their bores. This is not visible with the naked eye but it is there. When a FT lifter fails to rotate for various reasons is when the cam wipes a lobe.
If you ran a roller lifter against a flat tappet cam with the same spring pressure the flat tappet cam would surface crack and flake away very quickly.
The other issue you'd have running a roller tappet on a flat tappet cam would be a sudden loss of power.
When replacing lifters, it is best to replace the whole set and they must be the same type as the ones you took out. As long as the used ones you put in are the same as the ones you took out and are in good condition there will be no problem.
I'm switching to Q&A format to be notified of your response.
It is a lot less work to replace a whole set of new lifters once than to keep tearing it down for another replacement. You will not be able to tell other than external appearance if the used one is good or not until after you install it.
Let me know if you need more help.