Usually for All Stars it seems they always include someone from every season, so then you would need 6 alliances of 4 each, etc.I think some of the uneven seasons is unavoidable as they can't force people to sign up and not every season has as many people who wants to play.
Having people vote in also prevents a lot of the "perfect set up"
I'm not sure how 4X4 is any less a set up than vote people in. After all they could guess that people would vote season 6 in, but they couldn't assure it. I'd have rather seen all the guest voted in rather than part, but that is me.
I think the adding two more houseguests than planned was because season 6 dominated the voting and they wanted to make it harder for them by adding more people so they weren't as overly represented.
I suppose it is the producers fault that season 6 keeps winning HOH?