If one tomato had molded, is the rest of the pack safe to eat?
Yes - the mold is an indication that the spores have entered that tomato, but do not indicate any problems with others. Mold usually enters fruits like tomato through the stem site or damage to the skin. The bits you see outside the fruit are actually the fruiting bodies of the fungus (equivalent of the bit you eat on a mushroom - the rest is below the soil). These fruiting bodies produce tons of spores. You should use the others fairly quickly before any released spores have a chance to potentially start growing in them.
Edited to add:
The general advice would be to discard any fruit that are attached to the main one by the fungal body, wash the others well to remove any potential spores off them, dry well (wetness promotes fungal growth) and use within a short time frame.
The USDA has some good advice here (PDF) - with thanks to SnakeDoc for finding this one.