This is such an obvious idea, with thousands of years of historical precedent.
What do you do when you're fabulously rich, but not very military, and suddenly find your planets faced with invasion?
There should be Legions for hire, with colorful names & perhaps some special attributes.
"Frogstar Death Legions" - a 5 Legion group that fights like 10 Legions
"The Immortal Legions" - an 8 Legion group that recovers 50% of its original fighting strength the turn after a battle (if it is not wiped out the first time)
"The Golden Legions" - a 10 Legion group that costs money every turn. If you can't pay, they leave.
"The Defense Legions" - 3 Legion groups that can only become a Garrison & cannot be transported after being hired.