Miku, though... granted, I've never played the game but she really comes off badly to me so far. For those who have, are those sore-losing recations normal for her?
Miku can become confrontational and rebellious when she doesn't get her way. The best example being her Master Collection drama track. I don't think any other idol got away with abusing/ignoring Staff-san that much, without getting pulled off the air or punishment trolling.
Isn't extreme stubbornness a stereotypical trait of house cats?
Sachiko is definitely the sore loser of the rivals. She's more... It's hard to explain, but she's more of a tease towards the producer.
Sachiko is egotistical and has a scathing tongue when talking with others. Sometimes she can be a "stealth insult-er", using nice words when describing something/someone, but with a ugly hidden meaning. On the flip side, she will sometimes use insulting words, but actually be praising you (mostly applies the Producer; there's some tsundre in her). The key factor is Sachiko never lowers herself, or raises another person's status above her. She's always the Queen Bee in her little universe. The more Sachiko likes you, the more Sachiko will let you bask in her presence and praise her. To Sachiko, that's the best honor anyone else can have... because no one else can ever be as great as Sachiko. Whenever she screws up, she always makes up excuses or justifications why it wasn't really her fault.
There are of course, curve balls in her character development though. For example she's very nice to younger children, has a strong sense of repaying debts, and karma that always bites her in the ass.
If I was forced to pick the "sorest loser" personality among the CG's... IMHO it would be Reina Koseki (she'll prank you for revenge)... or maybe Mayu under specific circumstances (that yandere jealousy of hers).
Also, I don't know why, but Anya and Minami work pretty well together. Maybe it's the whole "big sis/foreigner" match-up they have that just works so well.
Call it bias, but I think Anya + Ranko works together, being that Anya tends to mimic other idols under the false impression it's "ordinary Japanese behavior" she should try to adapt, and Ranko is one of the last persons anyone should think is normal. Plus given the "language barrier" Anya suffers from, she will probably never fully decipher Ranko-speak. So much gold comedic potential lies there.

according to the fever Friday night page and the wiki. It's Fujimura Ayumi, the same VA that do Veteran Trainer voice.
All four Trainers will have the same VA. It's all part of their character trait of being so physically alike. The four could have passed for quadruplets, if they weren't born in different years.