Those products are quite expensive and you can achieve the same result with leaves, wood, alder cones, indian almond leaves, etc.. You can also put some spagnum moss in a net and place them in your filter. Cheap and makes your water as black as you want.
If you ever go past snowplanet, the carpark there is jam packed with alder trees (where alder cones comes from) that you can pick at certain times during the year.
On the Geos my experiences is that a tank your size would be too small for a group of G. altifrons (surinamensis) and if you only had 2-3 aggression could become a problem. With S. leucosticta (jurupari) however (like david suggested) you could easily house 4-5 in that tank. They are smaller and more peaceful and get along well with all other fish, even the small ones.
Having said all that I think your idea of smaller fish only is really good. Have you considered hatchet fish?