Qccowboy:
sorry for the late reply, I was kinda busy and couldn't read thoroughly throughout your reply
Concerning the flute range, I will check the whole score again, and remove the notes that are below range (that is, below middle C). I was not planning on using an Alto Flute, now, but thanks for the remark
About the flute trills, I guess you're right. I just wanted to make sure that the trill is being heard. Well, when I get some time, I will have another instrument do the trill, doubling the flute, and will write some more interesting - harmonicwise - flute parts (as you yourself said

).
Concerning the orchestral planes of sound, which Adler analyses as well, I believe this comes with experience. I haven't studied lots of orchestral scores thoroughly (in fact, I've only studied Bartok's "Concerto for Orchestra", and that's because it was mandatory for my music course. Still, we analysed it more in terms of harmony and form, and not so much in terms of orchestration), but I will do that as soon as my exams are over, and I will try and write some other orchestral pieces, to practice. I will also read over Adler's book again and again, as I didn't even read all of it this time, I just read parts that I thought would help me write the orchestral part of the concertino.
Thanks a lot for taking some of your time commenting on my piece, it really means a lot to me

Take care, and see ya around here!