I think Imogen & Penelope go really well together (and I personally ADORE Imogen, by the way), but because Zachary, Alexander and Thomas are a little more mainstream they go well together but not as well with the girls. Overall though, all of your names are classic and nice so I think it makes a good sibset. It's clear you have a slightly more traditional taste in boys names, though. That being said I don't think someone who doesn't spend as much time as I do thinking about names would notice that.
Side note: I like that you spell Tom "Thom". It would throw people for a loop I think but I've never understood why we suddenly take the 'h' out when we shorten the name, shouldn't Tomas -> Tom, Thomas -> Thom. I've always liked Thom better.