The Aston Pocket works very nicely on the Umlindi. My general purpose back pack is an Umlindi with an Aston Pocket. The pocket adds more organization than it does capacity, if that makes sense, but it is a worthy addition.
You said "the Aston pocket mates nicely with the Umlindi, the Tara looks to be too small,correct? "
I'm not sure if you were asking if the Aston Pocket mates with the Tara, or the Tara mates with the Umlindi, so I'll answer both.
The Tara mates very nicely with the Umlindi. I don't carry it that way frequently, but I have, and the two together can haul quite a bit of stuff. You just need to be careful not putting anything heavy in the Tara, as it can be awkard having weight way out there away from your back.
I've never mounted my Aston Pocket on my Tara, but I think it can technically be done. There may even be some pictures of people doing that on the Facebook group.