SC2Mapster

Trigger would go like:

Event:
- Every 15 seconds
Action:
- Spawn 10 Rebels at SpawnPointA for Player 1
- Pick ever unit in (last created units) and do actions:
     - Order (picked unit) to attack SpawnPointB
- Spawn 10 Marines at SpawnPointB for Player 2
- Pick ever unit in (last created units) and do actions:
     - Order (picked unit) to attack SpawnPointA

To be able to find any campaign units, you'll have to add the Liberty Campaign to your map dependency. I can't tell you exactly where it is (can't look it up right now), but it's somewhere under File->Dependencies or Map->Dependencies.
Once you've added the Liberty Campaign dependencies you might need to reload the map. After that you should be able to see and place all campaign units.

About your camera problem: You can fade the screen to black everytime you do a turn. This way people wouldn't notice at least. I don't know how to get rid of these small stops - or if it is even possible.