the photo cell should have a rating on it say something like
1800 watts tungsten. the photocell needs to go on the power side (line) not the load of the transformer

yes you can use the 1 photocell for your 600w transformer. The bigger issue is about using a malibu transformer in general. Low voltage landscape lighting has voltage drop (the longer the run, the more lights, the less voltage at the fixture). Malibu transformers do not enable you to properly compensate for voltage drop, as a results lights are often dim/uneven. If you get the chance in the future, try a multitap transformer--it has voltage ports from 12-15 volts so you can output at higher voltages to compensate for voltage drop. That way if your lights are dim, you can connect to a higher voltage tap on the transformer.

All professional installers will use a multi tap transformer and they are a key factor to getting pro results.

If you don't have a multitap (like a Malibu), then be sure to pay great attention to your layout to minimize voltage drop. We have info on layouts in our landscape lighting learning area.

Good luck.

Here is a link that might be useful: Low Voltage Transformers for Landscape Lighting

Yes, the pro results thing is VERY important.
For great quality landscape lighting, it's important to have a good power source. You can make a MALIBU work, you just have to concentrate on wiring it in zones. You should then test the voltage at each fixture (it should be between 10.5 & 11.5 volts for optimal brightness & bulb life) If you can achieve that, it doesn't matter about the transformer. Professionals use better transformers FXL, Vista, Unique, etc... because it's easier to adjust that voltage in the field without having to do work/labor.

GOOD LUCK

Here is a link that might be useful: Phoenix Outdoor Lighting Info