is english opening the most flexible opening of all openings.

No - it's the most transpositional, often Black determining that.

1.c4 c5 or 1.c4 e5 will most often leads to a pure English, though occasionally the former can lead to the accelerated Dragon Maroczy Bind.

1.c4 e6 or 1.c4 c6 will typically force White elsewhere. His choices will ultimately be a Reti or a transposition to a QP opening. Slav against c6 and QGD or Catalan against e6.

1.c4 Nf6 depends. Against 2...e6, you have the Anti-Nimzo or Mikenas-Flohr. You can also transpose to a QP opening. After 2...d5, you typically have an anti-Grunfeld, though can transpose to a Grunfeld. After 2...g6, there is no better answer than to transpose to a Kings Indian, either Classical, Saemisch, or Fianchetto, depending on move order. The Grunfeld can be prevented via 1.c4 Nf6 2.Nc3 g6 3.e4 and only then 4.d4. Note that 4.d3 is bad because the Bishop is still open. The Botvinnik setup is best when Black shuts in the Bishop, playing a reversed Closed Sicilian, like 1.c4 e5 2.Nc3 Nc6 3.g3 g6 4.Bg2 Bg7 5.e4! d6 etc.

1.c4 g6 - this is the most effective time for Modern players to play 1...g6. With c2-c4, where the pawn cannot retreat to c3, the long diagonal is weakened. Even Kasparov played 1...g6, but only against 1.c4.

Because of needing to know a ton of transpositions, I never recommend the English or Reti to anybody under 2000 over the board rating.

A 1500 trying to mimic Kramnik, Uhlmann, or Korchnoi is just ridiculous.