Most herpetologists and reptile vets advise against keeping more than one bearded dragon in the same enclosure; males fight and a male/female pair will breed continually throughout the year until the female is exhausted from multiple pregnancies and relentless advances from the male. Two female bearded dragons often get along, but the most important things if you intend to do this are quarantine and observation. Bearded dragons can become highly aggressive in a competitive environment and even females will fight.
Always quarantine a new reptile before introducing it to an established vivarium with an existing occupant. Even captive bred animals from pet shops can carry mites and internal parasites so err on the side of caution and set up a temporary vivarium for at least a month to make sure your new beardie is fit and healthy. If you're worried about parasites, take your dragon and a fresh faecal sample to a reptile vet for a professional health check.
Two baby bearded dragons from the same clutch should get along just fine until they reach maturity as long as they are roughly the same size and stature. Introducing two babies form different places could be problematic if one baby has already established dominance in the vivarium. The most important thing to a growing beardie is food and if you suddenly add another critter to its home, it will inevitably have a nibble of the new house mate. Observe their behaviour at feeding time to make sure one is not bullying the other and they are both eating well. If they do fight at meal times, try feeding them individually out of the vivarium in a designated feeding box and gradually re-introduce them to eating together.
Never house two males in the same enclosure because they will fight to the death. If you already have a male bearded dragon and are absolutely certain of his gender, the only reason you should introduce another beardie into the same vivarium is if you intend to have a breeding pair. In the wild, male and female bearded dragons only come together to mate and before going back to their solitary existences. Put a pair together in captivity and you are condemning the female to a relentless cycle of over-breeding and being harassed by the male in between pregnancies.
Unlike many other species of reptiles, two or more female bearded dragons may fight for dominance in a vivarium even where there is a male present in the social group. Adding an adult female to an existing group of two or more will undoubtedly lead to trouble. If you really want to keep multiple bearded dragons, housing a female with another solitary female is your best chance of achieving your goal, but be prepared for some initial fighting and displays of dominance. Arm waving is a sign of submission and females tend to display this behaviour to males. If you see one of your two females arm waving then it could be a sign that one is bullying the other and you may have to separate them.
Mixed age groups
Wild bearded dragons eat anything meaty that will fit into their mouths and that includes small lizards. Under no circumstances return baby bearded dragons to their parents after hatching because they will eat them. Big reptiles eat little reptiles and if you introduce a new, significantly smaller beardie to your adult's vivarium, chances are it won't last long and will inevitably end up as lunch.