Is it just your DH and DD that would have J names? I don't think it is too much if that's the case. Just think of the families where every member has the same initial (my DH is this way!).
They sound great together. It doesn't seem like you were "going for" J names; rather, it sounds like you picked names you love that just happen to start with the same initial.
Re: Too many Js?
oops just saw that it was DS with J name. I still think it is fine though if that's what you like!
Not unless you and DH's names also start with J!
Nope, I think Jack and Julia go great together.
Now Jack and Jill on the other hand...
LOL!