Mixing Bantam and Standard Chicks?

Just bought 6 bantam chicks from feed supply store. They had arrived at the store only an hour before I bought them, so they must be just a few days old. We intend to buy 6-8 more standard sized hens and raise them to live together. How soon would you buy the bigger hens? Should I wait a week or two to give the banties some growth and a little head start? The birds from the other store could possibly be a week or so older than our bantams. Advice and anecdotes appreciated!

 

Thanks Folly's Place! Space isn't as much of a concern as whether the size of the baby bantams compare to standard chicks. They are considerably smaller, so does it make sense to give them some grow time before buying the standard size chicks?