It still won't work you'll need to spend the money on a bigger tank. With some of the ones you've chosen here, you should start with a 55. Even though there are some that can be kept in smaller tanks but none of the above apply.
That's not completely true and most of us try to not recommend doing so. I've seen it go bad both ways
No the filter in this case isn't going to help at all. Even if you changed the stock a little, 8 will still be to many. If you can't get a bigger tank you should consider re homing all the fish and going a different route
Actually they should be 1 male to 3 females but even some males will need more than that.
To the op these may help ya out some on a new stock list
http://www.cichlid-forum.com/articles/cookie_cutter_20g.php
http://www.cichlid-forum.com/articles/cookie_cutter_10g.php