At the moment I can't decide if it would be better to do Sully x Gaius or Sully x Gregor, and have no idea who to pair with Tharja as Henry is going to Maribelle. No clue for Olivia either but I definitely want a strong physical Inigo. Can anyone help me out here?
Suly and Gaius becuse Gaius will pass down Galeforce and as for Tharja, you could always pair her up with the Male Avatar for two free galeforce units unless your avatar is female of course. As for Olivia, that would be a tough call because I do not know who your other pairings are.
Olivia + Chrom, Sully + Donnel, Ricken + Tharja are generally considered the best. Olivia gives more useful classes to Lucina than Sumia does, Donnels cap modifiers mesh well with Sully's, and Ricken gives Noire an insane magic cap.
For Sully, Gaius gives the daughter access to the mobile Pegasus Knight classes (and everyone's favorite skill, Galeforce). Gregor passes the Mercenary money saving, legendary weapon spamming Armsthrift. Both are good choices (but people would generally pick Gaius for Galeforce).
Pair Tharja with Ricken if you want a kid with a higher magic stat.
For higher strength, pair Olivia with nuts Vaike is taken uh... Stahl is a fine second choice though his class options aren't spectacular. Olivia already passes Swordmaster and Assassin though so no biggie. Donnel can pass the high strength Fighter branch, if you want more options.
It's not my first playthrough, it's my Lunatic run. I'm trying to optimize the children for StreetPass teams. Tharja x Ricken does seem like the smart way to go while Gaius x Sully seems like it'll give the most benefit.