Finch Compatibility

Question
I currently have 4 zebra finches, 1 society finch, and 2 orange cheeked waxbills in an enclosure measuring 3 ft high by 5 1/2 feet long by 21 in wide. The finches usually hang out in groups of two and the society finch and one of the wax bills pair off together. The other wax bill pretty much goes from group to group. Every once in awhile the finches will chase each other but the temperament in the cage is usually fairly passive. If I wanted to get different types of finches in the future, such as strawberry finches, do I need to wait until all my current finches are gone or can they be added to the current group? I don't want to upset the current balance I have now as everyone is healthy and seemingly very happy. The finches have bred in the past but have not done so in probably at least a year.   Also, how many finches is too many in an enclosure that size?

Answer
So your cage can hold between 4-6 finches comfortably. Strawberry finches are considered passive birds so you should be able to add them to the cage whenever you like. If you are worried about fighting, one thing you can do is add visual barriers to the cage. What I usually do is use fake plants and hang them from the cage ceiling or weave them through the bars (after they've been cleaned, of course).