Since the housing co-ops are grown by accretion each building will be built under a different cost regime. This will need to be addressed when creating budgets and setting rents. This is an exercise in cost accounting.
The general principle should be to allocate costs to a building that are clearly generated by the building. The mortgage is the clearest example. Some costs (administration) are clearly not generated like that and would need to be spread across the buildings. Others are not so obvious (maintenance). Cost allocations will need to be made as a result of experience.
One consequence of this is that it is possible that costs per unit (and therefor rents) will drop when new buildings are added to the co-op.