Guppies, unlike the other fish you mentioned, will always be at the mercy of some tail nippers. Don't put barbs, several medium or larger tetras in with them. Obviously stay away from several ambush predators. ;)

Unless they are quite small, stay away from cichlids, who have an endearing ( or is that enduring?) tendancy to spawn in aquaria. It is really cool to watch them set up a territory and care for their fry. What they may do to trespassers in that territory is not so cool.

Even a pair of Kribs would claim a lot of that tank. They would leave most livebearers alone, if the livebearers kept their distance. I have been dismayed though by what they can do - especially to other cichlids!

You know that schooling fish are best off in schools of six or more. You pointed out to us that similar Cory species will school.

While it is expensive, two schools of small tetras (king's tetras are one of my favorites) will give more eye catching movement that a dozen miscellaneous egglayers.

Ask yourself what you like from among the fish you see in shops, books and on-line. Then read up on them in terms of their ultimate size, behavior, diet and habits. The Baensch Atlases in your library or Google searches would be helpful.

In that light, I'm continually intrigued by how miskairal's bala shark (at the moment) seems to comfortably co-exist with male guppies. I wonder how long that relationship, even though bala sharks like veggies, will continue. (They are large catfish after all.)

There are also a bunch of fairly expensive aquascaping and aquarium decorating books around. Maybe search http://half.ebay.com/products/books/index.cfm
to see what is, or was recently, in print. Then go order them through your public library's interlibrary loan. They will give you ideas you would never think of, short of visiting an aquarium club's aquarium beautiful show in a mall.

Planning an aquarium is one of those fun things which can make the hobby so special. If those new residents are well researched, the delight can continue unabated.