I'd suggest looking up the basics of orchestration - i.e. instrument ranges - so you don't ask players to do the impossible. Then write the score out at sounding pitch; tell each player what you've done and leave it up to them to either say what transposition they want (if you're doing it on a computer, you can presumably do any transposition on request) or do it themselves.

You don't want to force a B flat clarinet player to go sharper than C major or flatter than A flat major for routine playing. And some apparently easy cantabile passages are impossible on some brass instruments.

I find playing arrangements dotless pretty difficult (you are asking people to memorize something that isn't really a tune), whereas jamming an accompaniment line of my own is quite easy. You might get better results by keeping the directions rather loose.

For the usual folky keys of G, D and A, players will need clarinets or trumpets in A to do anything fancy. These are nowhere near as common as B flat ones.