I imagine the video game they will play as a set, because that is easier and also how Christopher intended them to play. The quote Pydro posted is from playtesting, because we needed to make sure they weren't game breaking if mixed.
My personal favorite team is New Mainstay, Idealist and Writhe and original Dr. Medico.
Is there any fundamental reason why we couldn’t mix multiples of the same Sentinel? Say, just for the sake of argument…promo Mainstay, standard Mainstay, Writhe, and Dr. Medico? I realize that the team isn’t really the same in that case, but the rule we know is that multiple copies of the same DECK are not allowed. I don’t have the Sentinels setup card with me; does it specify each member by name?
When fighting La Capitan, who knows what temporal shenanigans might result!
Thematically is the main reason I can think of for why they can't be mixed as long as you have one of each of the four unique members. Though I imagine in play testing these groups were tested to be balanced themselves and not a mix of them.
At the start of the game, put the character cards for Dr. Medico, Maintay, The Idelaist, and Writhe into play, active side up.
Shuffle the hero deck and draw 4 cards.
Also, in your example of having multiple Mainstays, this would mean any card that refers to Mainstay would refer to both. Both would get the bonuses from Durasteel Chains and both would deal damage when Good Hero - Bad Hero is played. So there are some serious balancing issues with having multiple cards with the same name in play when other cards reference that name.
If you played with two of the same Sentinel you'd also negate any cards that mentioned the missing Sentinel - say, as in the example, you had Writhe, Mainstay, and both Medicos, you'd lose out on the effect of any card that mentioned the Idealist (or gained bonus damage through her or something). Possibly balanced out by getting double the bonuses from whichever Sentinel you had two of? :D
That’s interesting… That’s almost the exact opposite of my favorite team. I prefer new Medico and Idealist, Old Mainstay, and I super don’t care about Writhe. I only need him alive for his amazing passives. I never, ever, EVER use his powers. I just never see the NEED to.
Promo Writhe has a situational power, but it's really good when it is useful. It counters Dawn. It's also good against characters that can bring targets back from the trash (Matriarch and the Mask/Cohorts, La Capitan and her boat, The Chairman and his thugs, etc)
Unless you are facing a lot of AoE damage the Sentinels last quite a while before being the target of highest HP damage, they do eat a lot of lowest.
Original Medico can actually heal non-sentinel heroes so you can avoid the Sentinels having to tank, and also keep your lowest alive in bigger chunks than 1 healing.
New Mainstay is awesome because the irreducible damage effect doesn't have to deal damage to trigger, and thus let's you one shot low HP targets through reduction, it is a 2 damage base power with an awesome bonus thrown in. It stomps old Idealist's power. Also Mainstay is the only Sentinel with a static damage increase. It makes sense.
I was wrong on Writhe, I use original Writhe, I was thinking Adamant Writhe had the higher HP, I use the higher HP Writhe for his redirection, although there are matches I would take new Writhe because the situational aspect is in play (looking at you La Capitan and Chairman)
T.K. Thump vs. Block is the real decision. I prefer the reductino on the damage dealer over protecting the Sentinels a bit. Block for me is mostly a card for trying to -2 damage taken tank with Mainstay, and I'm not a big fan of that. I prefer Medico and Idealist to help a hero with higher HP tank, but mostly I don't set up a game for tanking, I find it much more fun to race the villain to the death than to set up a long game, I also find that you win much more that way.
So in the end "Idealist, I choose you!!" (seriously, T.K. Thump sounds like a Pokemon attack), and because I choose T.K. Thump over Block I prefer Old Medco and new Mainsay.
It can be necessary with some villains. Progeny, for instance. Or Plague Rat. In fact, that team is one of my favorite heroes for him, without the super-cheese-fest that is Fixer and/or Ra.