I know a boy called Zeppelin too! His name is spelt with one P though. They call him Zepy. I imagine that people would constantly say 'are you a Led Zeppelin fan'? Also I have a vague memory of Zeppelin being the name of a military aircraft. May not be a good connotation.
I know a Zenith, but she is a she and an adult- and, funnily enough, a hippie! Not that there's anything wrong with that. Just thought I'd mention it as you asked whether it was 'too hippy'. Oh I do love the NN Zen. That is cool! Could you use Zen as a first name? Zen Felix Kelsie....
I wouldn't worry too much that your friend just used River, because it is quite a common name now. River Felix sounds kinda cute. And I loved River Phoenix...gorgeous- and a vegan! Although if you did use River Felix, you would have to deal with every second person saying something about River Phoenix!
I agree with your Jonte comments, too much e sound.