First off, good luck! A concerto is a brave thing to have a crack at... I used to be the manager of an orchestra as well as playing in several, so I hope the following helps!
It might be worth your while trying your nearest music library and seeing if it stocks orchestral material. Get any piece, not necessarily a concerto but preferably one you have a recording of, and just have a look through the parts, then you can compare the individual parts to the conductor's score and to the recording and see what things like those double notes actually mean to the individual musicians. You'll also see things like what cues are marked in the parts and so on... a lot of composers put bassoon/double bass cues in the piccolo part, it sounds mad but it makes sense, the piccolo is often used straight after the basses as contrast so if you have a long period of rests it helps to know what your cue is and who's playing it, then you stand less chance of getting lost!
For the record - as a flute/piccolo player - you can get harmonics out of flutes but they aren't generally used for orchestral purposes, just for solo material, as you can only really hear octaves and even then they come out pretty quietly.
Also, watch the transpositions for those instruments not tuned in C. Many are - but for instance, a piccolo is a transposing instrument in C, it's tuned an octave above the written pitch in treble clef, so if you don't know about it, it doesn't look at first glance at a score as if it is because it's an octave. Same thing goes for double bass, transposing in C an octave down in bass clef. Orchestral clarinets are usually tuned in A but you can also get variations in Bb, Eb, and a few others. If you want to use an alto flute it's tuned in F. Brass instruments aren't usually in C either, nor saxophones. Watch your clefs with bassoons, they can play in bass or tenor clef depending on the range (you change clefs for ease of the player's reading.) Same for cellos, and of course violas come in alto clef, but I imagine you can write for them in treble too for high passages.
Beyond pitch and clef issues, as long as you know what the instrument sounds like you probably can't write much that'll be unplayable even if it doesn't quite sound how you thought it would - although do bear in mind that woodwind/brass players do need places to breathe if you're taking an influence from Bach and writing pages and pages of quick semiquavers for them! Circular breathing is not a skill most amateurs will have...
Oh, and doubling instruments. Piccolo is often combined with the second or third flute part - you are allowed to write one part for two or three instruments of the same type (eg, picc/concert/alto flutes, or Bb/Eb/bass clarinets)so you can have a third flute to add harmony and a piccolo for highlights and range but you write an interesting part for one player with two instruments rather than two half- and therefore relatively dull parts for two players.
Once you've written it, go to your nearest university and investigate the student music society. If you ask nicely they might well spare some rehearsal time to play your orchestral parts for you to hear - and don't be afraid of asking for their comments on the presentation of the parts, the articulation you've marked and so on. There's nothing better than giving your music to someone who plays the right instrument and asking what they'd like marking in (or out! if you've accidentally managed to write articulation that isn't physically possible or something) for practical, playing purposes.
Just make sure you don't ask just before they have a concert coming up and are pressed for rehearsal time, and be prepared to offer a bit of a bribe or reward. I used to run a student orchestra, and we did this regularly, mostly for music students in the university but for anyone else who approached us. You can offer all sorts as bribes - a few bottles of wine, or some cakes or something - or offering to help out with a concert for them, putting up a few posters or writing some programme notes for them or something. Speaking as an ex-manager I used to accept any and all help people offered with concerts, they're a big job to organise and publicise, especially when you're a student outfit without a budget!
If you can find a soloist, if you do persuade an orchestra to help you, it'd be a nice gesture if you offer them the opportunity to play it in a forthcoming concert, you'd get to hear your masterpiece played in public and they'd get a lot of marketing mileage about staging a premier.
Good luck!
Topic: Complexity Of A Concerto