From the picture, the seater is a second op. If that is the case, do I need another seater if I buy a second pen. Or am I missing something about the assembly?
No. You may need to adjust the seater for depth with different pellets, but that's no big deal.
Are there any plans to manufacture a pellet pen/seater for use with .25 cal pellets? I, for one, would make the purchase! I just bought a .25 cal break barrel rifle from you folks, for hunting, and this would come in very handy in the field.
According to US8291894 (U.S. Patent Publication No.) filed by inventor Billie John Barwick Jr., this should be possible. Ask Tom... (The blogmaster should know.)
Any problems with loading more than one pellet at a time?
Not really, it has a small restrictor at the tip of the pen that kind of "stops" the pellet before exiting. The slider also has a little indexer to give you tactile feel of how far you pushed it down and help prevent you from pushing too far.
how deep do you set the pellets?
In some cases you may want to push them down further the breach. An example would be Air Venturi Bronco. It's all about consistency so at least until it's flush with the breach but if barrel is loose enough you may want to seat them deeper. Need to experiment and see what provides the most consistent shot.
Until their flush with the breach.