Like if I decide to date Tom am I still able to date or fuck other (named) characters at the same time?

Yes, at this time going along with the love interests doesn't lock you out of other routes. Per the dev via discord conversations, there could be some unique events or dialogue in the future based on these concurrent relationships, but no barriers or single-love interest routes are planned, by design.

The dev also answered this recently here; Romance - Orveia community - itch.io