[Q]

A good way to enhance the neutrals is to give them a special racial technology with improved facilities (they can only colonies the few planets in their system), a colony module for all planet types, a special atmosphere converter ......
The best is that they have all this technology even in a low tech start game from the beginning.
Eventually you will still overpower any neutral empire but in the beginning it might be quite difficult.
And one more important point: make them refuse any treaty.