General

I suppose Pokémon is an RPG with more than 100 playable characters (150 even!), and that still works, so just choose the characters that look cool. You can always change your mind later and release them back into the wild.

Urgh, doesn't fill me with confidence that I will enjoy the games when/if I play them...

Just to add some context from Suikoden to what @Glen-i said, there were a number of times in those games where temporary members would join your party for certain story beats; I think there were times that as many as 4 slots (including the main character) were essentially determined for you, other times you might need a certain trio in your party when talking to a recruitable character to recruit them, and so on.  I'm not deep enough into Eiyuden to comment on how smoothly it's done it yet, but I'll be shocked if it doesn't use a heavy EXP multiplier like the early Suikoden games did, which made it a breeze to bring a new recruit joining you at, say, Lv 5, up to Lv 50 in just a few battles. This allowed for constant reconfiguring of your party and testing out new characters, which worked even better if you threw the new recruit in one of the back corners of your setup. This is also super important considering that mastering the game's battle mechanics is going to depend on having a setup which produces the most potential Hero Combos.  For Suikoden and Suikoden II, outside of story beat-assigned characters, I would generally stick to a main trio based on how the story evolved (typically MC and childhood/family friends), fill the party in a little with Hero Combos in mind, and then in the last slot or two throw in some of the cooler looking or heavier hitting characters. I'm planning to do the same here  Also just seen the full trophy list @Hero-of-Time, looks like it'll definitely take a bit of a grind (max out weapons for all characters). I'll be going for recruiting everyone but I think I'll pass on grinding that out  especially with how long some of the other mini-game related trophies will probably take! 

Finally tried out the demo: it's pretty fun! Only did Level 1-1 and half of 1-2 (I originally intended to stop at the end of 1-1, so there's some proof of fun) The controls need some time to adjusted to (as with most 3D platformers, admittedly), but it's definitely aiming for that sort of platforming that's fast and pleasant. Like, it feels like it scratches that "Super Mario 64" itch, but with moves that have nothing to do with SM64. Penny's grounded moves are a typical dash and a sonic-style spin-dash. Her aerial moves are a double-jump, an air-dash, and a weird swinging motion that serves as a longer-reaching double jump. She also has a regular attack and a spinning yo-yo attack to fend off penguins (and solve certain obstacles). Level design reminds me of Kirby & the Forgotten Land: very linear, no camera control, some set pieces here and there, but there's always space for occasional exploration and side-objectives. Feels like a fast-paced platformer in 3D, done right.