The general advice when it comes to Element is that if you have to have only one, it should be Light since most of the Raid Boss EQs in the entire game up through Ep.6 are weak to it, and because Falspawn can appear in most, if not every, Quest. The Pets you should have more than one ELement for are the ones that are typically meant for fighting bosses, like the Fox one (I don't remember its NA name).
For a Summoner, it really goes down to your Sub-class as to which Mag to get; by default Pets use TEC, so the easiest choice is to go with a TEC Mag, but you can use MEL or RNG and use the appropriate Switch Skill from your Tree; no matter which PWR you use, keep in mind that a Pet's attack will still count as TEC-based for any Skills on your Sub-class, but not as a Tech. A DEX Mag is only useful for a Braver or Bouncer.
And just as a general FYI regarding DEX, with any Pet or Weapon above 7*, DEX does nothing to raise your minimum damage since they automatically raise your minimum damage to its the highest it can be without a 100% Crit Rate, so it is essentially a waste to dump any SP into DEX on your Tree.