I have a 28 inches diameter nozzle from an atmospheric tank with not really a big diameter (5 meters) and 6,35mm (1/4") thickness. In this case, can I analyse this nozzle through WRC-107 or I must use API-650?

So your "tank" is about 16 feet in diameter? API-650 Appendix P states that its applicability is for tanks in excess of 120 feet. Unless your project specifications actually call out which methodology to use, I'd lean towards WRC.

I would like to go with WRC and FEA,