You would have gotten better response but you made a couple statements as fact without evidence: 1) Nick Symmonds was a 800 meter runner. He was not a 400 meter athlete nor was he 1500 meter athlete. Symmonds was a top 100 800 meter person.
Guys like Rudisha ... . What does that mean? There is no evidence Rudisha would struggle in 1500 meters. As for 400 meters, Rudisha is relatively good at 400 meters. He may be a top twenty-five Kenyan 400 meter man.
Muscle fibers: Differences in human muscle fibers are slight. The reason one person cannot run a sub-90 second 400 meters and why someone else runs sub-55 seconds 400 meters is not necessarily a muscle fiber issue. Was Miruts Yifter, a sub-49 second 400 meter man fast twitch or slow twitch? Is Eliud Kipchoge, with his sub 3:34 1500 meters fast twitch or slow twitch? Running fast, 100 meters is powerful strength and neuromuscular coordination. Running Marathon fast, sub-2:08 is based on sub-4:10 one mile ability which is a combination of speed, VO2 max and anaerobic fitness and pain tolerance. I used 100 meters and Marathon as extreme examples. Obviously, the difference between 400 meters and 1500 meters are more subtle.