Aster has the most solid Roster out of all current Commanders
As such, he gains the least from adding Mercenaries to it
In general, your Early-Game Resources are better-spent furthering your Build, rather than purchasing expendable units
Losing Units also wastes XP gain; Spamming Cheap Hexite Ones will result in less XP for the Army Core
That said, Aster's Veterancy System grants extra mileage to Mercenary Factions
Keep in mind Merc Costs influence Veterancy gain; More units per purchase = Faster Veterancy
Aster's lvl8 +15%HP and Movement Speed bonus is also very useful for Mercenaries
He benefits from:
Reliable Early-Game Tanking (Valkaru Hounds, Cha'Kru Clakjaw, Marran Pulse Skimmer)
DPS Infantry (Can be loaded into Striders; Flame Walkers, Chas'Ki, Shock Troopers)
More Accessible Healing (Gensis Core, Cha'Kru Cultists)
Support Spellcasters (Arandi Chronowarper)
To complement Aster's functionality, I recommend your equipped items to be:
Valkaru Archeoalloys (+1 Armor to Mechanical Units per level)
Aster possesses many Mechanical units (Predators, Titans, Striders, etc)
Remember that Armor does not apply to Shields
Enigmatic Capacitor (5% Faster Ability Cooldowns per Level)
Aster has the most Cooldown-Based Units (Aster, Commandos, Predators, Griffins)
Large power-level boost for Aster himself and the Predators
Gensis Core (+1hp/s Regeneration outside of Combat per level)
This item has low utility until level 4 sadly
Useful for survivability, ensuring more units gain the Vet3 Bonuses
Can deprecate the need for Griffins depending on Build and Micro abilities 3.2 Free Corsair Mercenary Outpost
Affordable DPS Option; Okay Durability
Low Damage, Fast Attack Speed; Good Clean-Up (Killing Low-HP Enemies) when Pushing
Two Mechanical Units that will benefit from Valkaru Archeoalloys and Vet3 Drones
Corsair Rovers provide a good early-game tanking option; However, they cost 50 Flux
Corsair Raiders' Thieve Ability can greatly speed-up Early-Game Ramp-Up
Caution: Enemy Harvesters do not spawn on every map