Really doesn't matter. You just need to have at least 1 card that is large enough and able to load the dag alone. I have 6 rigs this is functioning on, all were 6x Rx470 4GB rigs, but then I took out a card in each and replaced each slot with an Rx580 8GB once I discovered the fix. Distributed the set ups so every 4GB rig has at least one 8GB card to load the dag. I have not confirmed if an Nvidia card can assist another Nvidia card in this way, I don't any under 8GB however the even stranger thing about all of this is that a p104-100 8GB will assist my Rx470 4GBs and they will mine.
You will need at least 1 GPU to load the DAG plus the Windows data system files on the GDDR. But you will still need to have at least 4GB for the other GPUs. What I mean is that you can't have 1 GPU that has 8GB GDDR and have the rest being 3GB GDDR GPUs. It won't work I tried.
The way the Claymore software works is that it needs to load the entire DAG onto each and every GPU. It doesn't just load it only on the first GPU. You can look at the resource history monitor and you will see that the memory for each GPU is completely used up. So this will still only work with any GPUs greater than 4GB but anything 3GB or less is obsolete in mining ETH right now.