The only reason that it turns out bad for them in stores is because they don't have enough water changes. Since you can do water changes for them every day or every other then nothing bad will happen to them. You can use the .5 gallons if you want but trust me, it's going to be more of a hassle fitting it all in there or using extra heaters. Breeders do this all the time.
Also use the lids that the cups come with so they can't escape, much easier to work with
As far as tail types, that's just it, they're tail types. There's nothing different between a CT, a VT, a DT or DeT, HM and so on except that some have different ray counts. They are all the same species though. In my sorority I have everything from VT, CT, HM, HMPK, DeT and DT and they all do fine oh yeah and one regular PK too ^_^
And stress stripes are very normal, anything from moving to being introduced some where else, different water parameters, different food, different girls. I have one girl, actually my DT Phoenix who always gets stress stripes at feeding time as does Cherry. it's their way of saying "you can go first, I don't want any trouble!" same goes for "breeding" stripes which can also act as submission stripes, just telling the other girls that she doesn't want any trouble! Baby Betta's can exhibit this as well.
And for the length of time, you are certainly over thinking this ^_^ I keep my girls in their QT cups for at least
2 weeks and that's with water changes 3-4 times a week (depends if I'm there or not) and sometimes I keep them in even longer! One of my girls; Lady, she was super feisty and had gotten out of her cup once when I didn't have the lid on and she started to try to attack her sister who was in another container. So the three sisters all had a vendetta against each other. So the smallest of the three, Jewel was the first to come out after 2 weeks then her sister Emma after 4 more days and then Lady came out a full week and a half later to make a full 3.5 weeks in the cup. I thought she was going to be super feisty still but the time in QT when she realized that she can't get at her sisters and the other fish made her calm down and she's still one of the sweeter girls in there right now!
That brings me to another point, sometimes you'll come across bullies like my girl Lady there and you'll see her picking on some other girls or just one girl in particular. If you see this continuing for a few days then you can scoop her up and keep her in either a clear breeder box or a cup like that for a week or so to calm her down. Basically they can still see each other but inflict no harm and after some time in there they realize how stupid it was and they kind of forget about it and then when you release they're just like "eh". I once did this to Stardust for 2 days because she had claimed the entire water sprite floating on top as "her" spot so in order to let the other girls actually swim around, I QT'd her in her cup for 2 days and let her out and she was totally chill!
But at the first two-three weeks you're going to see lots of aggression most likely, some flaring, biting, nipping, body slamming and chasing. This is completely normal and you need to let it run it's course. This is their time to pick their leader or Alpha and get things in order. If you disrupt the process it's going to take them much longer to get over things and realize that they'll be living together. However, that said, if you see one girl that is seriously ganging up on one other girl, you need to QT that bully and put her in time out so that she can see the girls but can't do anything about it. Make sense?