I created a boosted character using the token and decided to start the original story line instead of jumping to the Knights of the Fallen Empire. Can I still do romances this way or that will not work?

The story before the boost, including romancing your original companions, is not available to you. There may still be new companions that you can romance.

Using a boost skips your class story, companion conversations, and various expansion / story content (content that ties into the current story arc of Fallen Empire / Eternal Throne). You can never go back and do the skipped content on a boosted character. You miss out on any companion romances and titles that you'd normally get from doing that content. Boosts are not meant for new or newish players, they are meant for players who have already played through that content at least once.