Is making a starter a good idea when using a larger activator pack from Wyeast?
Should you smack the packet and add it to the starter, or add it after it swells?
Making a starter is generally a good idea
It doesn't really matter when you add the contents
It's the same process going on in the pack as in the starter. The smack pack contains some sugars and a little yeast nutrient, making it essentially a starter on it's own. It will get you to about the half number of cells needed for a 5 gallon batch.
The answer is not quite a "yes" or a "no". The smack pack contains a yeast nutrient that actually transforms the pack in a starter. You should let the pack swell before you use it, because that's the sign that the yeast has metabolised the nutrients, and, air being absent, has used the energy to grow in cell number. This takes about 24 hours. A Wyeast "propagator" is enough for a 2.5 gallon batch, so you need an extra "normal" starter to use it for a 5 gallon batch. A Wyeast "activator" on the other hand, is suitable for a 5 gallon batch without the extra "normal" starter. You can always make a starter from an activator if you want to use it for a 10 gallon batch, for example.