this is just my two cent but i beleve you can keep to species together aslong they are very differnt for instance, i keep red gold and impyean a pair of each in a pen20x35, and i have heard of people keeping firebacks and peacock pheasants pairs together but these are very different species they look very differnt they have differnt breeding displays andin the case of peacock pheasnts and fire backs differnt breeding and laying times. i do however agree that you whould never keep ringnecks and melistic together with or without hens they wil fight, nor should you keep gallos together , ruffs together, longtails together they will cross in a heartbeat and males will kill each other and in the case of mixed pens you should always do it with young birds and put them in at the same time and if you should lose one do not try and replace it if the others are good and established they will kill the new comer pen size is always crucial for mixed pens aswell