@heavanet- If possible, you should plan to pick your fresh, green tomatoes for pickling before they get too soft. I pickle green tomatoes every year, and the process of preserving them this way will make them turn a bit soft. If you start with tomatoes that are already getting soft, they might get very mushy or even fall apart after you pickle them.
A good tip that you can follow when choosing your green tomatoes is that they shouldn't be so hard that you can't dent them when you squeeze them gently. On the other hand, if they seem to be softer than a tennis ball when you squeeze them, they are most likely too soft for pickling.