Ant farm gel would be a better substituent for soil if you are planning to display it on the wall. It can come with variety of clear colors, light can pass through it therefore you can clearly see the frameworks developed by the ants, and they have no problem surviving in it. Looks great in any room and you can also install some LEDs to illuminate/accompany the color of the gel.
No, it isn't good for long-term keeping. I have kept ants for over a year- the gel feeds the workers but not the queen and larvae, who require protein in their diet and other specific nutrients. The gel is also likely to kill them within a few weeks. Stick to soil or no soil.
This is one of my black ant queen from this year:

She has about twenty workers. This species is a very productive egg layer and can be expected to start laying eggs within days of catching.
Lasius flavus, yellow ant:

L.flavus is much like the black ant, L.niger, but they are nocturnal and very shy. And yes, I am keeping three queens together. They can be polygynous if needed, meaning more than one queen. This colony has about 20 workers.
Messor barbarus, harvester from Spain:

These live in dry conditions and collect seeds with which they make a paste to eat, called 'ant bread'. Queens are very large, and they have a soldier caste capable of drawing blood in human skin. About 20 workers in this one as well, because the queen took a long time to start up.
Pheidole pallidula, mini army ant:

Very tiny, but very strong and aggressive. Queens are very productive. This species has a genuine soldier caste with a hugely disproportionate head. My colony has about 100 workers with one soldier.
Good site to get ants, German based company but fast delivery: http://www.ants-kalytta.com/home.php
Just remember that its best to start with the common black ant. Queens of any species take roughly two months to raise their first workers, and they shouldn't be checked on more than once a week during the process. Its quite interesting to watch.
If you need any help with the actual keeping of them or just advice, don't hesitate to ask.