We have different body dimensions. Cardio-wise, 200m swimming is roughly equal to 800m run. Mike Phelps certainly would under perform as an 800m runner, not due to muscle twitch ratio, but due to is less than usual body build. Almost no one gets a muscle biopsy before age 15. Universities or shoe companies or national athletic training programs usually order a muscle biopsy for athletes in their twenties. It's mostly T&F bro science to differ athletes by twitch ratio. No one is doing muscle biopsies on infants. We have no proof one is born with fast twitch. It is interesting that runners get caught up in twitch ratio more than do swimmers. In U.S., all inexperienced high school swimmers start as 50 yard free style swimmers. A 9th grader has to prove their ability as a 50 yard swimmer to be considered for other events. We have seen Miruts Yifter back in 1972 Olympics. It has been said that Yifter did a 48.xx 400m TT as soon as he got their in Munich. We all need speed. Was Yifter fast twitch or slow twitch? He also was a sub-13:15 5000m man.
We all do not have the same natural T-level. I would believe athletes with natural higher T-levels gravitate to power events and sprints.
Regarding Grant Fisher: You cannot prove he is better now because he ran 70 miles a week, according to you while he was in high school instead of a 35 or 40 mile per week 800m program in high school. If so, he would certainly be a faster 800m man and most likely a faster 1500m man today. You have no evidence Fisher is naturally slow twitch. I am sure he was capable of F.A.T. sub-52 400m in hs.
Regarding Webb: He should have stuck to 800m. You cannot prove Webb would not have thrived as a low mileage 800m man.
Jim Ryun, if born in a non-English speaking country would have certainly been an 800m man. Ryun, same height & weight as Duane Solomon had no business racing 1500m, 1968 Olympics, Mexico City. Ryun, I am certain would have been fit enough for Olympic gold, 1968 & 1972, 800m. I do not guarantee he would have earned gold. Eight-hundred metre races have a lot of elbow throwing. Ryun was clumsy and uncoordinated. He could have fallen in 800m races as easily as he fell in 1500m, 1972. Ryun should have been a low mileage 800m man.
None of your examples have changed anything I originally posted.