Define Activity Predecessors

At times it is needed in the workfow to make sure a step has certain criteria satisfied. Especially in business flow applications that have certain activities that can follow a specific set of activities. Lets say I have a list of custom activities created. Now in the workflow, I want to make sure that certain activities can only be placed after a shortlist of the activities from the whole list.

e.g : I have Activity 1 , Activity 2, Activity 3, Activity 4, Activity 5 and Activity 6

And I want to define that Activity 6 can only be used if the previous activity is either Activity 2 or Activity 3.

We need a provision in the activity as well as somewhere in the designer where a custom activity can be defined be valid only if it follows a certain list of pre-defined activities. And I want to define that Activity 6 can only be used if the previous activity is either Activity 2 or Activity 3.And if this can be achieved, can we have a validation at the publish step instead of the workflow instance failing during execution ?