Your current line-up [possible spoilers]

these are in my party

Oliver LV99: Dynamite LV99 , Sugarpea Sprite LV99 , Deecee LV92

Esther LV99 : Electogo LV99 , Aye Aye Sir LV99 , Tokotocold LV8

Swaine LV99 : Overripe Buncher LV62 , Sea Naiad LV99 , Greater Naiad LV99

.... um what would a good party be for someone who, say, just got Tengri?

quote DragonSlayer45

You could get Dinoceros, who is absolutely broken in both Strength and Defense. If you have no intention of breaking the game and making the rest of it a cakewalk, don't get him.

Other than that, most familiars you could get as soon as you get on the ship. By the time you get the dragon, you should be strong enough to get a:

- Hooray (NW of Summerlands. Really good attack, poor defense) in the ocean
- Lumberwood (Island SW of Hamelin. Excellent defense, even better than Monolith. It can also heal)

Just keep exploring around and taming familiars. There really is no ideal or better setup. Form your party how you want it.

... I like a challenge with my fights, so I think I'll wait on getting him till I've beaten the game.

the other two, however, sound like their worth checking out.

Oliver - destroceros, dynamite, sugarplum sprite

Esther - electrongo, grimray, fountainhead

Swaine - aye aye catcher, neolith, sunshade

Reserves - elegantiger, soldier bumbler, free spot

I may be a little foggy on the actual names, but my party and familiars consists of:

Oliver: Puss-in-Bouts, Napdragon, Pandragorer
Marcassin: Angelixx, Catastroceros, All-Seeing Idler
Swayne: Hurlabaloo, Triumphant, Neolith

I always have my little Mermite on reserve, as I just can't bring myself to putting him away.

This has been my favorite lineup that I've come across so far, Puss-in-Bouts is a powerful and quick hitter, while Napdragon has very well rounded stats altogether, and Pandragorer being the best healer in game (save for his evos, of course...) Catastroceros's speaks for himself in the means of raw power and defense, and Neolith has just nasty high Def... It's taken me through the end game and all the post-game bosses with no problems.

Also, maybe someone can confirm/debunk this for me, but I swear I've seen a Gryphon-esque familiar on Oliver in someone else's game. Am I losing my mind or is there actually one in-game?

Let's see if I can remember...

Cry-Baby Bunting: Dynamite - Burly Hurly - Puss in Boats
Shameless Flirt: Electrongo - Sugarplum Sprite - Beachhead
Han Solo: Aye-Aye Sir - Wayward Sun - Ghost Whisperer

Just got the opportunity to hit up the Tombstone Trail casino; I might swap something out for a gold tinman, because it's a gold death robot.