Super Magnets stack with any other items, yes: your example about the magnet and a bullhorn is correct (400m total collection radius would be achieved for personnel only).
No other items stack between different types though (only items of the same kind add up): if you activate 6 Bullhorns and 10 Metal Detectors you will have a collection radius bonus of 600m for Personnel and 1km for Money, even though the largest circle (Money) will be displayed on the map.
Let me know if that doesn't make sense or if you have more scenarios in mind.