This is an interesting subject as folks come down on both sides of the issue. Some will swear by pairs or trios while other will say spawning groups/colonies are the way to go. When it comes to zebras, as well as other plecos, I am a proponent of the group. However, due to the high cost of these fish this route may not always be possible. Assuming money is not a constraining factor, here are the reasons why I prefer a zebra group.
1. We all know that the smaller the female, the fewer eggs she will produce. However, there is a second issue. It takes a lot of energy to grow, and zebras are excruciating slow growers. It takes a lot of energy to make eggs. There is not enough for both in a younger fish. As a result the younger a fm fish might spawn, the more likely it is to stunt or slow her growth which also means smaller spawns for a longer time. In the wild or in a group setting the females will compete for the attention of the best males just as the males compete for the female's interest. Since the normal result is that the bigger fish win such disputes, the smaller/younger females are less likely to be spawning and thus the more likely they are to grow. In pairs or trios this is often not an option.
2. Genetic diversity can best be accomplished when there are multiple males and females which hopefully are unrelated fish. How diverse the gene pool will be depends upon how hard the fish keeper works to try and acquire unrelated stock.
3. Spawning is often contagious. In a group one will often get more spawning activity than if the group were broken into pairs or trios. All the competition and hormones in the water can be very stimulative. For example, in a tank of 12 breeders I have had as many as 3 spawns in various stages all in the same few week period.
4. In a pair or trio the loss of a fish can mean the end of spawning.
Of course this is all just my opinion on this topic. As Barbie has reminded me many times over the years, it only takes a pair to spawn
There are also some downsides to working with groups:
1. More fish = more potential fighting resulting in serious injury or even death.
2. A tank disaster can wipe out an entire group.
What makes the common person uncommon is common sense.