A prototype is a testbed and uses what is available because you can't power an aircraft or arm it with blueprints
You also can't do your initial designs with fantasy weaponry.
The A2 production series had 4 mg and 2 20mm cannon - this is the 'service prototype' aircraft - not one designed for flying around a test aerodrome. The only reason it did not have 4 cannon was simply due to availability of the weapon. As more became available the wing mg's were replaced with cannon and later as better cannon became available they started replacing the existing sets.
While a fast aircraft can use energy tactics it might not be able do it effectively. Lack of high speed elevator authority (in the case of most turn fighters), lack of high speed roll rate (once again common to turn fighters) and lack of armament that can take an aircraft out in one pass will all reduce the aircraft's effectiveness. The 109 series suffered from the roll and elevator problems but had sufficient armament to allow some energy tactics to be used but it never was the aircraft's forte. It was simply not maneouvrable enough at high speed.
The 190 was designed for the energy fight right from scratch and suffered none of the above problems.
The most common mistake people make when talking about energy fighters is equating 'fast' to being 'fast and at its most manoeuvrable at high speed'.