I've put together this article due to the high number of questions I see specifically about Scions asking for recommendations on weapons, unit sizes, and other miscellany. I've tried to pick out some of the better thoughts I've seen from the various Astra Militarum tactica threads, but I fully encourage anyone to edit and contribute as they see fit if there is anything I've missed..
The Tempestor Prime offers many similarities to the Company commander stat-wise. Instead of a default two orders and an invulnerable save, a Prime receives 4+ armor and the same aerial drop ability that Scions have, allowing him to keep up with them when they deploy mid-game. His wargear loadout looks very similar to a Company Commander as well with a few changes. Gone are the Boltgun and Shotgun, and instead the Prime is given the option to replace his shooting weapon with a Tempestus Command Rod for 5 points, which gives him a second order.
It is worth mentioning that the Tempestor Prime can take relics, so he could take the Laurels of Command or the Auto-Reliquary, if desired, for greater return on investment via orders. The Deathmask of Ollanius could also be an interesting consideration to improve the survivability of this otherwise very vulnerable character.
Scion Unit Size
Here we will cover the relative pros and cons of popular unit sizes.
+ plasma saturation (1x10 is 1 plasma pistol, 2x5 is 2 plasma pistols)
+ morale resistance
+ overall squad protection (forces someone to try to optimally split fire and so over/underkill is more likely)
+ easier to deep strike 9" from an enemy
= more detachment slots consumed per points
+ order efficiency
+ stratagem efficiency
+ special weapon protection (5 bodies to a plasma weapon vs 2 bodies to a plasma weapon)
+ more Scions in fewer drops (since you can only deep strike a max of half your units)
= fewer detachment slots consumed per points
Their weapons include the standard Astra Militarum as well as a Hot-Shot Volley Gun. The AM weapons still suffer from the same benefits and drawbacks as they do for other AM units:
Plasma Gun: Kind of too good of a weapon that overshadows the rest for how cheap it is. This is good for anti-tank, anti-MC, anti-Terminator, anti-character, and GW sells resin 5-packs of the weapons if you want more of them. There is not really a significant downside, and with the Take Aim order, you can reroll 1s to help mitigate the supercharge risk.
Meltagun: These aren't horrible weapons, but they're a little expensive and so close range that they're not very useful right out of deep strike. That being said, they can be very devastating if dropped out of a Valkyrie as you can still move closer than 9" after deploying from the flyer's special rule. But that's something you can do better and cheaper with AM Command Squads.
Flamer: I can't find a use-case for these on infantry. They have 8" range, so you cannot use them on the turn you arive from deep strike. They automatically hit, so they waste the BS of Scion units. They might not be a bad selection for an AM special weapon squad similar to the meltagun, but I can't justify using them with Scions.
Grenade Launcher: They're cheap and they have 24" range, but these are really not incredibly useful weapons, especially since Scions can start within Plasmagun rapid fire range of an enemy unit. The Hot-Shot Volley Gun blows the frag mode out of the water, and plasmaguns blow the krak mode out of the water, and Scions (and AM in general) are cheap enough that it is preferred to specialize units as much as possible.
Hot-Shot Volley Gun: These are actually well-pointed, and have a decent effect. They're the only heavy weapon available to Scions, but in 8th edition that's not nearly as punishing as it has been in previous editions. Even at 4+ to hit, 4 shots at 24" on the move can be useful, particularly in an otherwise short ranged unit that needs to be at 18" just to fire their small arms. The extra point of strength over the regular hot-shot lasgun is just icing on the cake. Most cases will find the plasma gun still better, but this is a great second choice and could even wind up in first if GW ever does anything to make plasma less appealing.
Melee weapons: I put this as a catch-all category for all the power weapons available to Scions. I have never heard of anyone using these because Scions (and AM in general) aren't really great in melee. I don't like them because being in melee generally means that you're not shooting, which means that you're wasting the real power present in a Scion squad.