so im moving to a battlefield using my 1 might recruit token unit. on that battlefield lies two enemy units whom currently controls that battle. One enemy unit is 5might and another enemy unit is 3 might. I play imperial decree. Can i do combat damage first to the 5 might unit?

Yes — your Recruit's 1 damage can be assigned to the 5-Might unit, and Imperial Decree kills it: the Decree only cares that a unit took damage, so 1 point is enough no matter how big the unit is.

Lethal-first only constrains you when you have enough Might to actually reach lethal on something — with a single point of damage you can't reach lethal on either unit, so you're free to choose which one takes it (CR 467.2.c.3, 467.2.c.7).

Worth noting the trade: the defenders sum to 8 Might and all of it has to land on your lone Recruit, so it dies too. The 3-Might unit survives, so the defender keeps control of the battlefield.