About whether to level each kingdom to 10 first or keep them all on roughly the same level... Definitely to about level 4 or 5 it's more worthwhile to keep them roughly equal. After, the decision depends on what you value more:
- Stat bonus that helps you fight
- Income (from tributes), more specifically the gold key & gem income you get from getting multiple kingdom tributes.
General consensus here on the forums is that the first is more important (especially on the four magic kingdoms, Darkstone, Karakoth, Burning Lands & Zhul'Kari), because the chance for multi-kingdom tributes isn't enough without three gold-star bonus on kingdoms to be worthwhile, and before you hit that point you will easily have had enough gold to get everything to 10 anyhow.
There are a few players that keep all kingdoms at level 9 to intentionally keep their teamscore low and thus the difficulty of their opposition in PvP, but I think you lose more than you gain that way.