perhaps cartoonsmart.com would be of some help?
if your not into video tutorials, the id probably recommend either republicofcode.com or theflashconnection.com
both sites have some interesting tutorials that will help you im sure. just to set you on the right path. the first point you made in specifics about creating enemies that walk at different speeds. id do something like this:
speed controls how fast it will move.
x1 and y1 control were it will be created.
have a movieclip in your library with the graphic of your
enemy in it. right click it then go linkage. press export for
actionscript and then were it says class type "Enemy".
enemy = new Enemy();
// now by adding this array we can refer back to the enemy after its been created
here we code how the enemy will behave.
for simplisty we'll make it move left until it goes
off screen then we'll reset it to other end of stage.
thats just a small example. i hope it helped.if a class had been used it would have things a little easier. but i guess this was enough for now.