Yes.
There are no limits* to how high a pixie may fly, provided that they end their turn only one square above the ground. However, if they do end their turn above their altitude limit they fall with the consequences laid out for falling (1d10 damage/10 foot of drop).
*The theoretical maximum height a pixie may reach on their turn (with no extra movement trickery) is 8*3 + 1 or 25 squares high (run 8, trade standard for another run 8, action point for another run 8, +1 for starting height), that's 125 ft at the end of which they would fall. Falling says to subtract the creature's fly speed so that leaves us at 95 feet of fall, meaning 9d10 falling damage.
Not really. Most things work normally, except for the issues that the articles in the other answer mentioned. The primary one of those being that you can't take full round actions if you have a minimum forward speed (aka: average/poor/clumsy maneuverability and do not have Hover). Beyond that, flying is considered movement, so for combat purposes it's treated that way except when the flying rules say its not.
For ranged weapons specifically, altitude differences should also be factored into your range increments. After all, if someone is directly 50' above you, they're not in the same square! (A table here mentions that issue, squares while flying are 5x5x5 cubes.)
Unfortunately, calculating those distances accurately in three dimensions isn't all that simple. If you want an accurate measure, you can use the Pythagorean theorem. That is, if an enemy is 25' away horizontally and 20' above you, the distance is this:
x^2 = 20^2 + 25^2
x^2 = 400 + 625
x = sqrt(1250)
x = 35.355339 (so 35 after rounding)
You can imagine how doing that at the table would get tedious pretty fast, but I'm not aware of any rule they came up with to simplify it in three dimensions.
(If you find the flying combat rules overly difficult, KRyan made a set of variant rules that address this and several other complexities of flying which might be of interest.)
Best Answer
Gliding is not a thing in 5e
And in fact, Gliding is just a form of flying (just with less effort). Colloquially, we envision gliding to not take movement, but in 5e Movement is simplified and generalized to mean distance traveled. From the Basic Rules:
Your fly speed as an Aarokocra is 50, so any time spent "gliding" would still use up that 50 fly speed.
Flying is falling and missing the ground
When Flying, you've basically got a choice between flying with movement, hovering (if you can), and falling. Gliding isn't an option, so it must fall within one of those possibilities and you probably don't want to be falling.
If falling, you can reference this question for information on that.