Vcc and ground PCB layout for a 60 watt single layer LED panel
I know that if I decrease the clearance between GND and VCC is the same as adding decoupling capacitors that will help in case of power supply voltage fluctuations.
If the GND and VCC are on the same layer the capacitance will be too small to matter, the plate area is small. If the GND and VCC are on adjacent layers, then the plate area can be large enough to create a usable capacitor.
I can't be sure, but it seems that you haven't used the recommended footprint for the LEDs. With the recommended pads, the solder surface tension will hold the component in place. Without a proper pad to contain it, the LED will wander in the molten solder.
I'm not sure that I understand the second question, but the answer is probably no.