Yeah, but if you have Fire powers already, having nothing but Fire type Pokemon makes the Pokemon seem pointless. :/ All four of my characters have pretty balanced teams.
Tim is a child of Kyogre, he only has one Water type (Ludicolo) on his team (Three if Castform rain and Rotom-W count, but they can also be other types too.)
Mari is a child of Meloetta, she has two Normal (Togekiss, Wigglytuff) one Fighting (Breloom) and one Psychic (Wobbuffet)
Scott is a child of Shaymin, he has zero Grass Pokemon and one Flying type (Yanmega).
Selena is a child of Azelf, she only has one Psychic type (Musharna)
And really, miles#, Darkrai and Cresselia are supposed to be rivals, not downright arch enemies. Stop being so extreme about it.
If one wanted, they could even be quarrelling lovers.
*shot* Or siblings, that's the direction people usually go with them.