Much obliged for the thorough reply. I'm doing fine with the battles so far, because I'm willing to grind a bit, I abuse the bersek mechanic to speed things up considerably. I'm gonna go with the Seijiro route, because honestly he's hotter than everyone else... but who should I pick as my second companion for first time? I'm thinking Mia, because she's got First Aid. I would like to go with Seijiro and Kippei and get some hot sandwich action right off the bat, but not if skipping endings will harm my experience.
Seijiro is West Wing route. It's longer, harder, buggier, more tedious and obtuse than East Wing. So, its not recommended for the first go. If you do go with West Wing, Kippei is actually recommended because his extra backpack space makes West Wing much less tedious/obtuse.
(I once got softlocked in the middle of an event on West Wing and lost a save. Be careful of interacting with a doll in one of the save rooms. It can ruin your save file if it softlocks. Just remember to rotate saves as a security measure.)
Picking Mia and Yachika for East Wing route is the best first run because it lets you have a very strong first go at it. Which will make all future playthroughs significantly easier. Mia's
arguably the closest to a "canon" romance for Homura because that's Mia's horror trope. Those two also have a few exclusive-CGs that can only be unlocked if you pick them both together.
All the non-Homura playable characters have a horror trope going on and unfortunately there's just not much to them beyond that. Mia is the timid love interest one. Kippei is the comedy relief one. Yachika is the slutty bitch one. Seijiro is the jock/delinquent one. Rikana is the occult-nerd one. That's basically all there is to all of them in the entire game summed up in 2-3 words each.
Which is also why Harem ending is the only ending I don't really recommend going for. Its the hardest/most annoying ending to get and none of the player characters are worth it, lol. This game's interesting bits are really all in the setting, story, and the antagonists.
Rikana's got a little more going on than the others because she has some lines that arguably play into the overarching story. So, going Rikana ending is recommended at least once. There are actually I think 2 or 3 notes in the entire game you
need Rikana's dictionary to read, too. (I know one of them is just very, very well hidden.) So, Rikana is the story character, basically.
Do I get to change my party after the first ending, or am I just permanently stuck with it? If I can switch it around, I would like to run Mia for the first time, and then maybe someone else the other times once I know what I'm up against.
The only times you change your party are when starting a new run or when one of your current characters gets perma-dead'ed you'll run into a random member of the other group to join you. (If there are any members of the other group still alive.)
Mia basically becomes useless after the first couple runs because you get to keep items between saves and items will make her redundant. Which is why Mia on the first run is a good idea because her First Aid means you'll use fewer items on your first run, giving you more items for future runs and your item stocks will start snowballing in size between runs faster.
Only reasons to bring Mia along after the first run (If you got her ending on the first run) is for the necklace subplot, any CGs of hers you missed, or bringing her along for the extremely brief interaction Yuki has with her on True Yuki route.