I confirmed this morning with a representative of the US broker and the broker website, that FIFO rules apply to the same currency pair AND the unit size (lot size) of the trade.
I specifically asked, as a follow up question, the example of opening a trade for 0.02 lots on a currency pair, and then opening a trade on the same currency pair for 0.03 lots. The representative confirmed those trades are independent of each other and the FIFO rules do not apply to those two trades.
US FIFO rules only apply for the same currency pair and the same unit size.
Let me know if anyone understands differently.
Thank you!