I think you're a tad bit confused, I don't recall ever hearing Sully x Gaius being optimal, if I remember correctly the best father for Kjelle is Donnel, then again I haven't been on these forums in a while, so I could be outdated.
Olivia x Virion is a very good choice if Chrom is already married, or you just don't want Olivia x Chrom. I use it personally over Chrom x Olivia and I am very content with the Inigo I have.
If those are the only six people you have left, then I do recommend just doing the former.
Lucina has the best Physical Potential with this. I...... beg to differ which i will explain later
Virion is a rather garbo daddy..... Except for Inigo!
Everyone is good with MU daddy, however Sumia gets extra point for surprisingly nice mids and ironically her so-so skillsets
Galeforce, Sol, Armthrift. Troubadour did not do much, really
Infinitely better mods than Donnel, missing only Galeforce.
Vaike goes to Sully Sumia goes with Chrom Olivia goes with Virion
Vaike Sully gives everything she needs as a General. If you want Galeforce on Kjelle, its fine. Sully can also become a pretty nice Hero. Also you can try to pass Axefaire for some lulzy stuff
Sumia Lucina makes a "just enough" speed for Palladin Lucina, giving pretty much the same offensive power as Assassin and Great Lord Olivia Lucina. However, Sumia also gives option to make Tank Lucina AND Magical Lucina. Sumia Lucina is pretty underatted, but Olivia is considered the best for a good reason.
Inigo is basically the same as Chrom version, minus Rightful king and Luna. In return, Inigo gained Sage Inigo as an option for maximization