Are you looking to make this standard legal? Are you planning on playing this competitively or just casually?
I'm not really seeing why you would want the first three creatures in the build... Goblin Fireslinger, Gladecover Scout, and Llanowar Augur don't really fit in here.
For a discussion centering around a slivers deck that is standard legal, look at this thread:
http://deckbox.org/forum/viewtopic.php?id=12209
If you're going modern, the Mirror Entity route is something that is used, but typically in EDH as opposed to the more conventional constructed competitive formats. In either event, if you're not playing EDH, you would want more than one in there if it's part of your main strategy.