this... is a fun little question

ardbert is, at his core, an extremely good-with-people kind of person who can pretty much get along with anyone- his companions were all very different people, after all, and he loves them all unequivocally, even Cylva. so I think getting along w/ the scions would be the easiest thing he's ever done, and he would likely adore the whole lot of them- if they were his people, they would be /his people/ and he'd be a steadfast companion for every single one of them. the twins especially are so similar to him when he was young- leaving home too early to go off and be heroes without any idea what they were getting into, and the entire learning curve of trusting other people and realizing their failings. I think he would adore Alphinaud for this, and Alisaie's protective rage speaks to him on a personal level. had he the chance to be their friend, I think he'd very quickly become Big Brother to them both.

that said- man, i just think his relationship with Urianger and Thancred would be 100% a mess. even /without/ the fact that Urianger directly worked with him in Heavensward and the whole... Minfilia Situation in ShB, Urianger's tendency towards lying and aversion to direct, honest communication would be vexing and frustrating to someone as honest and genuine as Ardbert- especially because in all other ways he would /absolutely adore Urianger. that's best friend material right there. bookish, withdrawn, hard to get close to and always a little bit somewhere else- sounds a whole lot like Nyelbert, his best friend from the First, and exactly the kind of person his charisma and extroversion would be tempered and quieted down for, lending to more personal and deep connections. ...but the lying!! the secrets!! just talk to him man, they can figure something else out!!

this would only get worse with getting closer to and further understanding Urianger's motives and personal strife about the topic- like, argument level, "can't you see we love you can't you see we're hurting for you too please trust us with the person we love" sort of arguments.

....thancred.

thancred.

thancred would be messier. thancred would be less "i adore you in every way why are you doing this" and more "WHY ARE YOU DOING THIS". it's all the communication problems of Urianger without the reasons he can understand and the personality he adores- a good drinking buddy, a trustworthy friend through and through, but he would not accept Thancred's unwillingness to communicate NEARLY as gracefully as the WoL does in ShB. there would be a LOT more arguing with Thancred, at first asking and then demanding that he become better than this- an anger sourced out of love, of course! but that's the kind of anger Ardbert feels most keenly.

a mess.