The BTU-input size of the burners matter, as well as which end of the run the piping run the tankless lives. Typically the lines inside the house are on the low-pressure side of the regulator, and there are charts for calculating how far you can run for so many BTU.
At 10 feet you might be able to get away with half-inch with a 180,000 BTU tankless if you keep the radiant heaters off when running at max flow. Some tankless units are pretty flaky when fuel-starved, so whatever the BTU charts say, give it some margin. At a typical 2-2.5 gpm shower flow it'll be running under 100,000BTU/hr, but cranked to the max to fill a tub you can hit the high-range pretty easily. On page 14 of the manual it looks like you could get away with up to 20' of half-inch, if you keep all other burners off while running it, but I wouldn't recommend pushing it that far.
Ideally the tankless would have a dedicated gas line back to the regulator rather than having it on a trunk line with tees to the radiant heaters, in which case you may be able to run them all at once, with minimal interactions.
If you can find a YouTube instructional on installing tankless units (or better yet a manufacturers training seminar through a distributor) it pays to study up a bit before diving in- it's more than just another plumbing job- it's a bigger (and more sensitive, since it's modulating) burner than most people would have for their space heating furnaces & boilers.