You didn't mention an OS, but, for Linux the generic solution is Wonder Shaper for BSD you can use dummynet, for other OS's, other solutions, google and find them ;)

Depends on what kind of latency you're worried about and how you measure that. Yes, traffic shaping can improve latency of one application's traffic vs. another's, but if you're talking about the ICMP ping latency displayed in the graphs here, of course it will suffer with any kind of traffic on the network card or interface, no matter how you shape your traffic. But ICMP is probably not your bread-and-butter application and therefore that latency is not very interesting to improve. If you are experiencing serious problems with performance of real IPv6 applications it's another story of course. Applications that would noticably suffer from 200ms latencies are real-time two-way voip streams.