to create new material indices, make the material you want, on a plane or whatever, so it’s handy, then, go into edit buttons, go into edit mode, select the vertices you want to change the material of, and,…
in the edit buttons are some buttons relating to materials, that look almost identical to the vertex group buttons, which are right next to them. in the materials groups buttons, press ‘new’, then, go into the materials window, and change to the cork material that you made. now, go back into the edit buttons, and press ‘assign’.
for your other question, just go into the edit buttons, and use the ‘decimator’ slider. it may not be pretty but it will lower the polys to whatever the required range is.
<edit> btw, material indices are covered in the documentation.