Impact on genetic diversity with 6 person mating groups
Context
I know enough about genetics to know I need to ask others. I'm writing an alien humanoid species that revers the number 6 as part of their culture and religion. Their marriage equivalent forms polyamorous groups of 3 males and 3 females (lets call them units for now) where members can reproduce in any combination they see fit. Children are then raised by all unit members.
Question
I am intending the species to stay small and to grow very slowly if I can avoid bottleneck issues, even into a space faring civilization. How many children should a unit need each year/lifetime? How would these units improve or worsen their genetic diversity? How small an age window can we have possible unite mates? What might I have overlooked?
What I hope will provide enough information to answer how careful they need to be, ask if you need more.
1. For the sake of this question they are similar enough genetically or mimic the same processes we do for reproduction so we can use our genetics to determine answers.
2. The culture is not developed enough to understand genetics but will recognize that visible traits are inherited, they will likely account for a history of sickliness or deformities when choosing a mate.
3. 759
Concerning possible mates they do not allow children of a unit to mate with any member of said unit even if they are not blood related.
4. Most will form units with others of their same generation at the age of consent. It is very rare to not form a unit with people out of their same generation (within a few years).
5. They have a very narrow period they can conceive yearly that cannot by realistically adjusted. However their chance to conceive is very high so it is unlikely they will not have children if they try. This means there is a regular influx of new members of the species all within roughly a months time each year.
6. A unit is under no directive to breed if they do not want, but they can have children for a roughly 20 year period.
7. While I will be tweaking numbers we can use and assume normal human lifespans and calendar for discussion where I have not specified otherwise. 1008