elemental siginificance is similar to the build/destroy pentagram/circle that you see in 5 phase theory.

so a water sign would hold up a wood sign while being stifling to a fire sign.

a metal sign would cut down a wood sign, an earth sign would smother a water sign and so on.

worthy of note that you generally, as a rule of thumb can't have 2 dragons in the same house. It's in teh nature of dragons to not want to share the horde with another dragon.