A railgun is part of a system, component parts of which have only been tested across US and CN ships as I understand. A 130mm gun I don't think accomplishes much other than dick waving, the US "desire" to acquire one might simply be more to factors such the lack of an intermediate gun within the calibre range.. the warship picture you shared I think was intended to act as some form of offshore artillery in a bombardment role, akin to the Zumwalt class in an obviously smaller capacity.I don't believe the 054B will feature a railgun. I even doubt any surface combatant would have that in the next two decades.
However, the 130mm gun on the 052D should be perfectly suitable for a 6000+t frigate. I don't see why you think it's a green sea combination...
Whilst it is certainly be possible a 130mm gun "works" on 054bs, what would the justification be for doing so? Simply adding more dakka to a ship to please enthusiasts accomplishes nothing, at the cost of leaving less money available for actual and useful upgrades and optimisation available to other vessels, operating within the confines publicly understood.
PLAN has its own set of requirements I don't believe in any way influenced by such ideas, and despite disappointing legions here and elsewhere they arent building 20x CVN's and 055's, witnessing the launch of numerous new platforms and systems is far and beyond cool to a pedestrian such as myself.