lol you're facing THE hardest problem in OFP editing, one which is largely overlooked because the average mission maker is barely capable of understanding that it's a serious problem .... where to place the units.
I would suggest that you get the mechanics of the mission sorted out first ... location, where the chopper will actually be able to get in and so on. The civvies and the enemy can be fitted in later, because you can be relatively flexible about that.
As a general rule, units should never just stand there. (The occasional sentry being the commonest exception.) They should always have waypoints. In general, they should have a switch trigger putting them into different waypoints/behaviour when the enemy is detected or some other appropriate event occurs.
It's a big city. Some soldiers will be on patrol, some will be in base. Some of the patrols will have vehicles, some will not. You could have a 3 phase attack on your position. Phase 1 - local patrols, no vehicles, light weapons, easily beaten off. Phase two, disorganised vehicles arriving from around the city, greater numbers but still light weapons. Phase 3, organised response. Slow to arrive (so you might be gone) but with light armour, lots of machine guns (NVGs if appropriate) and snipers.
Key thought: life should be going on as normal in the city before your abrupt arrival. So create normal life in the city, or at least an OFP representation of it. Then create the response.
Are the civvies genuinely neutral, predisposed to you, or predisposed to the enemy? What happens if you shoot one? Shoot several? Are they mixed up with the enemy? Or is it civvies to your north and enemies to your south? Does detection by a civvy count as detection by the enemy?
Before you are detected it is usually better for the enemy to be in small squads (3 or 4 loons) because that means you can spread them around more. Once you are detected, it is normally good to have them in larger squads: once a small squad has taken a couple of casualties it is no longer a squad, its just one or two blokes, frequently fleeing. Consequently, one of the things that happen on detection is that the smaller squads join up into larger ones.
I'm using "detection" as a generic term here, in your case it may be the chopper crashing or some other event. The important thing is that there is a decisive moment (or several moments) in the behaviour of the enemy AI.
Whatever happens, you will need lots of playtesting to get the balance right.
Since civilians are to an extent cosmetic, you don't want any more than are absolutely necessary. You also only need them where the player is likely to see/encounter them. I think there is plenty of scope for variety in a mission like this: some run towards you, some away from you, some get into cars and race up and down the street. Maybe if you shoot too many they get guns and come after you. Maybe if you get to a "safe house" they will fight on your side.
A point worth considering is good miliary tactics. For example, if in the real world it would make sense for the player to control a particular piece of ground or line of sight, make sure that it is actually to his advantage in the game. For example, in one of my missions there tends to be a bunch of enemy AI milling around the main street of a one street town .... looks a bit crap sometimes, but the point is that if the player has the sense to take up a fire postion controlling that street (which he can easily do) he can wipe out a significant chunk of the opposition without any trouble.
But, in the end of the day, all you can do is experiment. Get your mechanics and principal location sorted out and just try putting loons in different places, doing different things. You may have to can that whole location and start again somewhere else if it doesn't work. Experiment, experiment, experiment.
Last point: plot. Imagine that its not a mission you are designing, but you are writing a short story. What happens? How would you describe it? Who are your main characters? Where are they at the start? How do they relate to each other? How will they react to the news? How will they get the news? How fast will they react to the news? It sounds basic but you do need a beginning, a middle and an end. OFP is flexible so you can have several possible ends. Or even several possible middles. What ends and middles do you want? Well that defines your beginning.
If your mission wouldn't make a good short story, it probably won't make a good mission.
I've written all this and still said nothing about placement of units. Except that, once you have answered all the questions above, plus all the others that you will think of yourself, a lot of the unit placement will become obvious. I hope.