There are also a number of considerations one needs to make when considering "exotic" aquascaping. Obviously the first and foremost consideration is how it will look.
I personally would not recommend building a pillar like structure in a tank less than 18 inches wide, and even then, 18 inches is probably on the small side.
For a pillar to look effective it needs to have sufficient space around it to grow corals and allow fish to swim. The pillar itself needs to be substantial as well. So assumiong a pillar is 6 inches wide you are left with 6 inches front and back if it sited centrally. If you place it more towards the back then there will be no space for the fish to swim or for corals and it will be very hard to light correctly (lighting is the second consideration here). If you do go with it towards the back I would personally make it an arch as opposed to a pillar. This has advantages over the pilar (see below).
The aesthetics of the pillar also need to be taken into account: Its not just a case of adding rocks, the pillar needs to look natural, which means it should be larger at the base than at the top. If it is too larger at the base it runs the risk of losing its "pillar" status an become a mound of rock. I experimented with structures coming to the very top of the water. I liked the look, but lost space for corals. So I figured the best height for the pillar that would allow sufficient growth of corals was 10-6 inches below the surface.
My experience working with 18 inch wide tanks is that the pillar is best sited approximately 2/3 of the way back and should have any elongated side run roughly parallel to the front of the tank. It is difficult to place anything behind the pillar without them coming together.
On a 24 inch tank, i would still go 2/3 and have rock behind the pillar
Now to arches. This works really well. If you take a pillar like structure and cap with a large piece (ideally long and relatively thin) you can make an arch that connects the top of the pillar with the remaining rock work. This has advantages in that it stabilises the pillar and removes the requiremnet of growing corals on the back side of the pillar. The top should be selected well to allow space for corals to grow (ideally a flatter piece works well) and should have a slight angle upwards towards the back of the tank. This makes an excellent showcase for the corals on it. By having a pillar central(ish) to the tank and the cap stone being at an angle it will make the tank seem larger as it opens the space for the fish to swim.
Finally........decoration of the pillar. To my mind, the base areas of the pillars are best suited to flat corals such as montipora capricornins. The sides are best suitd to tabling corals and pretty much any frags you may have. The top is suited to staghorns and other finger shaped corals that require high light intensity and good flow.
--just as side note, if you decide to go with any pillar like structure, be carefl to not place it dead center, or it will look artificial.
Good luck with your aquascaping....I offer the above just as ideas. They are by no means the only way to do it but are just my thoughts on how.
Regards,
Paul.