Im a few hours into Baldurs Gate II right now. FIrst of all i have not been this engrossed in an RPG since Fallout 2. This game is amazing! I rolled a ranger character since thats how i usually go in RPGs. My party thusfar is as follows
-Me (level 8 ranger, 18 strength, 16 dex, 16 con, 11 int, 14 wis, 8 ch just off the top of my head, might be a bit off)
-Minsc
-Jaheria
-Jan
-Valygar
-Yoshimo

There is an oversaturation of ranged characters in my party. Both me and minsc are more than capable of handling a blade so i usually use them as melee characters, but they are even better at standing in the back with compound bows. Valygar and Yoshimo are more or less useless but i use them as my rangers for now.

What i need is a good tank character. Where can i find one? also is Jaheria a good spellcaster? so far shes worked really well for that role, but im wondering if shell be useful later in the game.

Jaheira can tank. Give her a club or scimitar, you can put heavy armor on her and give her a shield, and her Dex is decent. Tanking isn't the best choice for someone who will be combat spell-casting a lot, but I mostly use her healing (after combat spells). Her hit points aren't ideal perhaps.

I don't know of Jaheira is overall a great spell-caster, but she does have one of the most useful mage-killer spells there is- Insect Plague. I'm playing through Throne of Bhaal with her now, and she earns her place in the party with that spell alone.

Minsc can tank of you go mace / shield with him, but you can probably find some nice two-handed swords for him, so you probably don't want him to go the tank route. Then again, it's possible to get a hold of decent maces, you might eventually want him using a mace anyway.

I have used Mazzy as a tank (Umar Hills quest is where you come across her). If you put a shield on her, she can't use all those proficiencies she has in short bow, but you have other archers.

Anomen can tank (Copper Coronet). If you want to try mixing an evil character into your party, Korgan is potentially your best tank (also Copper Coronet).

Keldorn could tank (Sewers), but he's built to be using a two-handed sword, and you'd be sacrificing that to put a shield on him.

Korgan, found in the Copper Coronet, can make a very good tank. He's evil aligned though, so may not fit in with your party that well. (I use the mod that allows the NPCs to get along, so it's never an issue with me).

Your title asked about mage too.

Of course, the story so far revolves around you rescuing a mage.

Early in the game, multi-class mages can serve you well; later in the game, their mage class level will lag too far to give you access to the highest level spells when you need them.

Then again, particularly in the later game (and probably often early), you'll find that you really need at least two mages. At least one pure mage with one multi-class mage can probably satisfy that need.

If/when you rescue Imoen, she can be that pure mage. Your alternatives are Nalia (Copper Coronet) and Edwin (Shadow Thieves quest)- Edwin comes with the problem of being evil if you have a good aligned group.

Then there are a few multi-class possibilities You have Jan, he's a great option, and covers thieving adequately too. I like Aerie (Circus quest), the mage/cleric combination gives her such a wide assortment of spells for so many situations. You can't go wrong with either.