Correction: But not low-magnetic field Tokamak. High magnetic field Tokamak are a lot smaller and could fit on a large spacecraft. They're room sized, not skyscraper sized.
ARC has a mass of 7,190 tonnes (not including generators or radiators). This is about the mass of three US WW2 destroyers. It's not room sized; the thing is something like 20m tall (once you include the support structure).
A 190MW(e) fission reactor would be considerably smaller. There is really no use case for DT fusion in space, especially if they have to make their own tritium.
Pretty much all space-faring SciFi, including "hard scifi", is basically garbage that you can toss out the window as far as expectations go. Think things are hard to cool on earth? Space leaves you with radiative cooling only. The least efficient (space x time x cost x performance) type of cooling in the known universe. Internally-powered propulsion in spacecraft is never going to happen with thermodynamics around.
