Okay, my party as it stands is this:

Archer
Tashia
Kelsey
Anomen
Keldorn

I have two tanks, however, I could use one more. I also feel uncomfortable with 2 Mage types, for some reason. I'm just better with three. Therefore, the logical conclusion would be that I would want Soulafein.

Er... Not exactly. I've played with him too many times, and eventually, I feel the need for something fresh. Haer-Dalis also fits the bill, but he doesn't gain enough Mage spells.

Anyone have another idea on what to do? Stick with 5, add Imoen (but having 2 frontliners... Just say taht I'd prefer not to), or just go with another tank?

As you obviously have some very good people in your party (alignment-wise) it would be harsh to reject Imoen. However until then or if you choose to ditch her.
If you want a tank:
1. Mazzy: Can be used as tank and as support (with the Machinegun-Tuigan) and has nice interactions with the other Paladins.
2. Valygar: Backstab! NIce equipment for start. Gets on my nerves from time to time
3. Minsc: Tough and cool but I have the feeling somehow that he doesn't fit into your party as you have nobody he has a lot of banter with.

For a mage:

1. Jan: Brings fun into the party and is a more powerful mage than most want to admit. I concur that he is only multi-class but inspite of this drawback he rocks. Also he is a thief and you are missing one right now.
2. Aerie: For the full spell selection. So you'd have 2.5 mages and 1.5 clerics.
3. Nalia: With a lot of potions she can do the thief tasks and you'd have another straight mage.

Dude, where's my Thief? I never go without a Thief. Never ever.

Yoshimo -> Imoen.

And between Anomen & Keldorn, to whom were you planning on giving the Gauntlets?

Er... It goes to Anomen, because he needs the AC, as he is the Tank. Thieves are fun, yet there aren't enough variety of them. I'm tempted to run my own Assassin, but... I like NPCs.