Well, the best conventional tank sort of character is Valerie since you can armour her up and she's built for that type of role and the tower shield specialist starts getting all sorts of bonuses to defenses when using tower shields, including bonuses to touch AC which will be more important later in the game. She'll take to hit penalties from the tower shield until level 5 of the tower shield specialist though, but it might not matter much on the easy difficulty level.
Brilliant, this is bound to help me out, any other tips for having Octavia, Linzi, Valerie, Amiri and Tristian in the team? Thanks in advance.
Well, since I assume that you're new to Pathfinder and probably D&D, when you level up, there are icons indicating feats that are recommended and not recommended which can help you level up. You can also choose the auto level up. If you'd like to look around at more options, then go ahead. I think it's best to get an idea of what each class and prestige class does and see if there's anything that you want to aim for.
Before that, I'd probably just focus each one on the class they start with to get a feel for what they do.
Octavia is a rogue/wizard that can easily move into the arcane trickster prestige class which will help her with both magic and rogue sorts of activities. Otherwise, I'd just level her up as straight wizard.
Linzi is useful continuing as a bard since that provides a lot of skills and buffs. You'll probably want to focus on raising charisma.
Valerie's usual path is tower shield specialist and/or stalwart defender. Personally, I prefer keeping her as tower shield specialist. You'll probably want to raise strength and dexterity.
Amiri is a barbarian and continuing along that path will help by giving her additional rage abilities and duration. You'll probably want to focus on raising strength and using two handed weapons.
Tristian is set up to be a healing specialized priest and is very good at that. You can focus on raising wisdom and should try and keep him out of melee. You could focus on ranged or reach weapons, but probably most of the time, you'll want to use his spells and special abilities.
There are all sorts of things you can do with those characters and most of the tips in these threads are about the more unusual things you can do, but if you're just learning the system, it's easy to become overwhelmed. So I would recommend trying to just play them as given at first and getting a feel for the game and system. If you want to try other things after that, you should. If you have the Beneath the Stolen Lands DLC, it adds a large dungeon that can be played in a stand alone mode where characters level more quickly than in the normal campaign. This can help you try out new ideas and see how other classes work.