A large colony dosen't realy need slaves but it's more a thing of interest to have them and they help a bit. A small colony is more likely to do well if it is boosted with slaves.
To give slaves to the colony you can go out and find formica colony and collect lots of brood in a small container. You can't let some ants go into a nest and just steal brood because they need to size up the colony an see what would be the best entances to attack through etc. You can tip the brood into the nest. To make things much more interesting, you can collect some workers with the brood and place the container in the foraging area and watch the ants coordinate a raid.
