Well, what I find most effective, is first think to yourself, can you see yourself in THEIR uniforms?
Could you picture an army dropping from the sky or gunships homing in dropping waves of kasrkin troopers into the hell we call home?
IF so, then look at this as well:
Can I see myself pulling the entire army though with no distractions from other armies?
Can I make them work to my own taste and tactics?
IF the answer for both was yes, then there is part 1 complete.
Now, in terms of the army, I would combine them! Paratroopers with grenadiers is one of the most efficient armies out there. They can drop in when called uponed and deliver a swift strike to the enemy, before drawing back and allowing the slower elements of the army to advance in and take the grounds. Combining them seems good however, if you are concerned about pricing, aim for grenadiers first as the platoons of troopers will cost you alot of money to put together. Grenadiers are cheap, hard to murder, and can swing around to hit the enemy in all directions.