I have a 12 month old, a 15 month old and a 16 month old on one day every week... getting around is tricky, so I keep things very local. They get on well though and all is going fine :)
I have 1 x 3 yr old, 2 x 2 yr olds and 3 x 1 yr olds... mixed and matchd throughout the week. All works fine, just needs planning and forethought logistically.
This is Monday to Thursday - Fridays I keep for my graphic design work... I need ONE sane day a week ;)