I've never done it, but I believe you can go 6 deep, not that I ever would. I am two deep at this time for one miner in one room just because I haven't pulled it out yet.
I added another switch and now can run the Avalon6 and S5 together for 4.7TH. Good thing it's been cool the last few days, I haven't needed to use my heater at all.
I don't know if I would base it on how modern it is. I have some from the past that are metal and far more substantial then modern power strips. And some modern ones it's kinda hard to tell just by look.
But I would not base it on how modern it looks. I have seen modern looking ones that are crap... so just hard to tell via picture. And even though they are crap for "non-miners" they likely work fine. Us miners use more electricity in a lot of cases than a typical user.