First-time investor here, what risks (esp tax) should i be concerned about if i am planning to get a rss plan with fsmone where I purchase Nikko STI ($200) and Vanguard S&P ($100)?

For NikkoAM STI ETF, not much tax risk as dividends are tax free for income tax purposes. Growth-wise, the SG market is flat. If you are looking for dividend income, you might want to look for ETFs with higher yields that FSMOne has.

For S&P 500, forex risks and dividend withholding tax of 30% applies. For long term, there is also the estate tax. The Seedly team has done an recent article on US ETFs. You can search for it.

The usual transaction costs and ongoing fees should be looked at too.