Yeah combat is pretty simple. Summons require a bit of work to obtain and use but frankly, they are not that useful.

Nikki is a potential party member but I'm going to slightly spoil something and say it is best to avoid Pierre for the moment. He's the game's resident Lethal Joke Character but only on a second playthrough because you need to do both paths to get all of his gear.

Leena is a play on the Alice deal but with an interesting subversion you'll discover later.

Also just a note, the epilogue scene is kind of a lie in one regard, while you are correct about the story part, your party is a bit of a twist because sometimes the game throws in characters you can't actually obtain by that point in the story.

Chrono Cross features several remixes from Chrono Trigger. For instance the world map theme I think your talking about that sounded familiar was Plains of Time which is a remix of Corridors of Time. Game has a few cool nods to CT like that.

Hope you are enjoying it, the game gets a lot of crap but I still enjoy it.