Eh, they come and go as they please. They're all pretty much G1 spawned.
1) Wheeljack had a glitch that prevented him from physically expressing emotions. The fins installed on his head were originally created to help portray emotions before his glitch was solved.
2) Reflector is a photographer, first and foremost, and as a photographer, he's absolutely fascinated by the dinobots.
3) Red Alert and Wheeljack use to work together as saboteurs. Red Alert took down security systems, and Wheeljack made the place fall.
4) It's difficult to tell the difference between a full grown Minibot and a youngling of the same model.
4a) There are always those stories with Bumblebee being a sparkling/youngling, but what if it was Cliffjumper instead? Do any of the bots know, or is it his best kept secret?
4b) Cliffjumper is a hardened youngling because of his crappy past and current predicament of war. He lives with the ideology that he doesn't need anyone to take care of him. But when he watches the older bots smile affectionately at Bumblebee, a part of him can't help but be envious.
5) Blaster was a sparkling/youngling care taker in the past, and those old protocols re-activate when he observes the dinobots acting like younglings exploring. Really, they were all just over grown kids.
5a) Don't forget the Aerial Bots.
5b) Ditto for Soundwave, and the Stunticons are seriously weirded out.
6) Visors were commonly used by younglings to hide their age, as optics are typically large before the final frame, and considered 'adorable'.
6a) Soundwave is actually 'big brother' while Frenzy and Rumble are his little brothers. The other cassettes are from the animal shelter he used to volunteer at.
7) Seekers aren't the only ones with protective sparkling protocols. All the older gestalts have it too strangely enough, and it activates when new gestalts are around.
7a) Devastator believes in tough love. Bruticus likes to cuddle.
7b) Insecticons also have these protocols.
8) Megatron wears a large bucket instead of his normal helm for some reason or another.
9) Sunstreaker anonymously posts humorous comic strips about bots from both factions in the rec room.
10) Perceptor is an exceptional gambler, especially in card games. He can count cards like nothing.
11) Coneheads damage their helmets and have to use standard seeker helms.
12) An annual 'Geek meet science fair' festival attracts the scientists from both factions. Cue idiocy when each bot submits their science fair project and tries to sabotage others, faction be damned.
Skyfire: Perceptor! I know that was you!
Perceptor: Skyfire, might I suggest Mixmaster as the culprit?
Mixmaster: It wasn't me- explosion in the back - That was me.
Wheeljack: If that's yours...
13) Primus is curious about humans, and when he investigates their world he comes across a whole slew of gods.
Random god: Hey, a newcomer! We haven't seen a fresh face since the martian bunch over there! Here for the Olympics?
14) The stunticons get into a Capella singing. And they're GOOD at it.