This is depending on your budget, but if you have some cash to devote to this....
The way I would approach is would be to fire a pair of CO2 jets down on either side of the rocket. Red and orange lighting, perhaps a single strobe mixed in with the solid colors on pulse to give it some pop.
The CO2 jets will give you the movement you want in the "exhaust" rather than just the nimbus which won't give the lift off effect you want.
If you REALLY have a bit of a budget, I'd do the nimbus as like a "pre-launch" effect and then fire the CO2 jets just after for the actual launch- will build excitement and more closely mimic a rocket launch.