it's realism when we have round planets in-game, just like robbie said, in a pure fictional universe, even the slightest derivation of pi would make all circles look like weird geometric objects, so we are based on realism for the most part
Those examples you mentioned are fiction based
on real-life science (hence the term science fiction and not the term universe fiction). Realism in those examples is more like technobabble as it ain't in my example with the green stars and water planets.
Btw. I can give you a logically coherent=science-fictionally realistic solution for your problems:
Extremely rapid travel between the 500 or fewer stars of the galaxy, but absolutely no travel in "normal space" between stars can be explained by the existence of dark matter that prevents space bending or warping from taking place (space cannot be bent if no equally large "light matter" mass is available between two waypoints, i.e. the planets and stars).
All intelligent species eat the same substance known only as "food": Now that's easy, just imagine food to be what each race likes. Where's the problem in that? Agriculture can be redesigned the way you want it to be.
Battles occur over an entire solar system, but relativistic effects and inertia is totally ignored: Dampen those out. As long as you don't need uber-energy to do that as you don't since the same amount of energy for accelerating (supplied by the star ship engine) is needed to dampen it's anti-accelerating inertia effects. Quite easy to make this work while keeping game balance logically. (that's btw. the important part about all realism or logical coherence talks. It helps in preventing to unknowingly create uber-style energy sources hence possible game-balance-breaking effects/weapons/whatever plus additionally, it gives you a nice descriptive text of those features for the game's encyclopedia for free so to speak
Each empire has instant communication with all ships and planets at all points in the galaxy: Communication requires minimum mass and energy transfered hence dark matter problem for considerably larger masses/energies like ships or even weapons(torpedoes) sent at uber-light-speed does not occur and fast travel of that information is possible (even faster than with the "massy" ships).
There will also be "monsters" that live in the vacuum off space and can travel the star-lanes: No problem, who says anything about or against biological engines? No one. It does not violate anything experienced in real-life since all kinds of life-forms including human beings are small little "reactors" producing energy all-day long.
Get to grips before wasting your time
. And hiding behind that link won't help you. You still do not understand that it's not so much about realistic "arguments" but about a realistic "description" and "origin of the idea" that I'm talking about. In other words, if I hadn't explained where my green star idea was coming from and simply said: "I want green stars on the map for all non-human races", wouldn't you all call me stupid without that explanation I gave you? Yeah, you would as I would have too probably.
Btw. I bet just a handful of people know about the non-existence of green stars. Who actually has seen that many stars in real via hubble telescopes and how many would have noticed there were no green images among them? Virtually none. So your last argument about people finding that weird fails here. I'd even say they find it weird over time that there aren't any green stars on the map!