Hi Tom,
The best way to work with Source Control (and that's true with internal and external work), is to split your project into as many Work Units as your project need. For example, in the Actor-Mixer Hierarchy, you may want to separate your structures into different work units (e.g. Weapons, Footsteps, Vehicles, etc.).
Also, if you have a lot of assets in a given category, say vehicles, you can create child work units for each vehicle. That way, somebody can work on 'Camaro' without conflicting with others working on different cars.
Hope that helps,
Simon