The answer to your question depends upon the target of your composition. Obviously, you can expect more from a flutist in a professional orchestra than one in a high school orchestra.
Assuming that you are talking about a pro orchestra, the typical range written for flute is C4 to C7. Pro flutists are often capable of reaching F7 (and in some cases even higher) but that is not typical for most orchestral literature. A few orchestral works will take the flute up as high as D7 but it is not all that common. If writing for less than a pro orchestra, then the range should be reduced accordingly. Regarding your passage up to G6 and G#6, that is perfectly reasonable and very common for a pro or even a good high school or college flutist.
Regarding your comment about pitches below C5, yes the lowest octave of the flute does not project as well as higher pitches, but it certainly can be heard unless the rest of the orchestra is playing FF. The lowest octave is often used for soft passages with solo flute.
Every instrument has some limitations and it is up to the composer to accept those limitations and how to use each instrument to it's best advantage.
"Never give a flute player a screwdriver."