What sample sizes are ideal for carrying out Bayesian Skyline Plots?
I found a PLOS one article that might help. In this they used similar numbers you're planning to use (if i get your question and the article's data right). 


  The influence of the sampling scheme was investigated by drawing 40 samples in three different ways: 1) all 40 samples from a single deme, 2) 4 samples from each of 10 demes and 3) one sample from each of the 40 demes in the structured population. These correspond respectively to the local, pooled and scattered 


(taken from the above linked article)

So depending on your sampling scenario your 30 samples seems to okay.

Also this is a link for the BEAST FAQ that might be helpful.