I would never try to clip fins on a fingerling. The fish are stressed enough as it is. More handling, loss of slime and risk of infection you might end up with <2% breeders.
Greg is dead on here. Just way better bang for the buck getting larger already to breed fish and tagging them so they don't come out on selective harvest. I planted 8" FLA lmb, 15 to be exact in the Spring in a 1/8 acre drawdown fire pond. By Summer I had numerous 1-1 1/2" fish. This pond dries every year. I just have to get them out now.
I doubt with your plan you will have any noticable or visible difference for years. But stick with it, on a five year plan and eventually you will have a large % of FLA strain genes in your lake. I might also make a growout pond or brood pond for pure FLA. Keep the females in your brood pond and dump the males in your lake. Grow fingerlings to 6-8 inches, dump them in. That's fast tracking on a budget.
