Time to pimp my own video, here is a video of me playing imperialist turkey against a rank 5 giving tips and tricks along the way. The game actually starts around ~9:30, It's slow and dry though, sorry.
Basically like W4r said, have a mix of tanks in with the infantry when you can afford it to balance out the lack of offense. Expand heavily as you will need the unit's to overcome your deficit in power as well. Other than that, try to be more indirect and not rush as much.
Its great that it reduces all your units cost but it reduces all your units attack by -1. So back to the question. How do you play imperalist?
Spam like shit. This is an ongoing casual game of mine where I'm playing as Imperialist Turkey:
At this point, I've won the game since there's about a 1000 units on transports and 362 bombers (used to be 547 - lost a few) invading America, so I've just set my cities to autoproduce infantry which I'm leaving in place/stacking in a few cities (100 here, 75 there) since I'm kind of curious to know how high I can get my unit count as full Europe Imp before going into the negatives. Point is, spam like crazy and win by drowning them in numbers. Papa Stalin would be proud.
Your strength lies in numbers and your capacity to spam more troops than your opponent. Gain spawn and use a combination infantry and tanks to attack and defend.
----
The enemy is in front of us, the enemy is behind us, the enemy is to the right and left of us. They cant get away this time! - General Douglas Mcarthur