Unless you are going to somehow stunt them physically (which does not change the genotype), you will need a dwarf mutation. That is a chance thing with any organism, so unless someone discovers that sort of mutation and propagates it, I think you are out of luck.
1 - expect .... THE spontaneous mutation one on millions of birth.
2 - a genetic transplantation ... long time .... lots of money !
In the genome of Chihuahua find the gene responsible for dwarfism and inject them into the genome of the peacock.