I'm not sure what the max sga size is, however 600mb for a shared pool seems excessive. I assume you have over 3gb of ram. I would suggest dropping the shared pool to 100mb and dropping the buffer cache to 2gb.

You may want to look at MTS. You need to understand how much sorting each session will do to know what to set the sort area size to. When you only have 20 session at a time you can make the sort area rather large and not think twice with 600 sessions and 1mb sort area size thats 600mb for sorting alone. You may want to drop sort area retained size so that session give up memory when they don't need it. But definately look at mts.