Will dog's which have an excellent guarding ability lose their guarding ability when it's deed (or fixed)?

No. If ur dog is trained to guard and isn't just barking and growling because of hormonal stuff, then after being fixed, he'll still know how to do what u trained him to do. And he'll also be much trainable, can't get testicular cancer, and is MUCH less likely to ever bite someone. Go for it! U'll be glad u did. I've spent my life rescuing dogs and as an atty know that over 90 percent of all serious dog bites are form unneutered males.