I am not your financial advisor. Be wary of anyone who recommends against diversififying your portfolio, as they either have an agenda, or have gotten their advice from a chain that begins with someone who does. In general, emerging markets are going to be a lot more volitile, and sometimes can crash entirely (examples include Venezuela and Sri Lanka), but if you invest smartly you should see average performance that exceeds a mature market like the one in the US.

I would recommend checking out robotraders such as Wealthfront, Betterment, Fidelity, etc. These services have algorithms that are more effective than you or I, and they can establish a properly balanced portfolio.

TL;DR: Unless you are a professional (which you are not, since you are asking this question), use a roboadvisor and don’t worry

Yes, I personally do 30%

There is a research paper by Vanguard titled "Global equity investing: The benefits of diversification and sizing your allocation"

On page 5, it shows that for a U.S. investor, the best amount of international exposure for ideal risk adjusted returns is around 25% of your portfolio. Based on their research, I think its wise to have at least a little outside of something like an SP500 fund.

Yes. I keep 25-30% in international stocks. FZILX in my IRA, I-fund in my old TSP, and whatever the institutional equivalent is in my employer plan.