You can do that, usually. Some conversation options are gated by your alignment, but most aren’t. As a fighter, there’s no penalty for changing your alignment.
Just level up everyone in their current class if you’re worried. The exception is Octavia, who you really want to make an Arcane Trickster. You can select it on level up screen and see its requirements, so be sure to work toward them.
The teamwork feats, Precise Strike and Outflank are great for your meleers (but no one else). Normally I would advise shying away from Weapon Focus because there are such a wide variety of weapons you’ll always find something better (but different) than what you have. But since you’re a fighter you want some of that sweet Weapon Specialization, which requires Weapon Focus. For the NPCs, I usually have them keep the weapon they come with so Weapon Focus for them is usually fine. The exceptions would be Amiri and Harrim who are always changing their weapons to whatever is best.
For Valerie you’re going to want feats to maximize her AC. So that’s Armor Focus, Shield Focus, Dodge, Greater Shield Focus. Probably a few others.
Improved Initiative is fine if you’re running out of ideas. Generally the recommended feats are fine, but sometimes it will recommend a ranged feat like Point-Blank Shot for a dedicated meleer.
More channels for Harrim and Tristian. More bombs for Jubilost.
Consider making Linzi a skill monkey. That’s feats like Alertness, Persuasive, Deceitful, Skill Focus: Perception, Skill Focus: Trickery, etc. She’ll never do any damage anyway.
Take Extend Spell for your buffers. The other meta-magic feats aren’t really necessary as there are items that grant a few usages of the feats that add damage.
Make sure everyone takes Blind-Fight at (or by) level 15. Because reasons. You can work around this, but it’s annoying.
There’s an NPC in Oleg’s (and later your tavern) that will let you respec if you mess something up.