Only the highest buff applies, so if you for example used BarThundaga, Love You All, and Hyper NullAll, the end result for your team would be:
+70% thunder resist
+50% all other resists
+30% total damage mitigation
From the units you listed, I'd put WoL, Refia, and Rikku on nearly every trial team as the core, which covers tank, break, heal, mitigation, and resist. For the two DPS slots probably Fryevia and Noctis.
Willhelm and Marie can shine in niche scenarios, but if you don't need ultimate tankability, or a minimum of 70% resist buffs, then both of them lose out to the wider versatility of WoL or Rikku. DKC (enhanced) is a far superior DPS than Noctis, but I think in general the emergency raise, breaks, cover, and just overall versatility of Noctis can be more useful in general (for trial fights).
Ling's your only unit listed to provide team mana sustain, so if you're going into a long trial where that's important, load her up with auto-refresh materia/gear and have her keep using mana dance.
For something like one shotting a raid for farming, I'd use WoL (def/spr breaks), with Fryevia, DKC, and some of your chainers.