An error occurred while saving the commentAnonymous commented
So apparently you can do this with the Advanced rules.
It may be helpful to add such a thing to the Behavior tab in the preference pane.
Otherwise, this is a solved problem.
Steps to do this:
Open Choosy preference panel
Select Advanced tab
Press the [ + ] (plus) button to add an advanced behavior rule
Press the [ + ] (plus) button to add a rule
Add the rule: Number of running browsers is 1
When this rule is used Choosy should: Use the best running browser