I'm sorry, I'm having trouble understanding your plan.
BTW, I'm sorry about the mistake I made concerning Imoen in my previous post. When you recover her from Spellhold, you want her to be an 11th Level Thief dualled to a 6th Level Mage, correct? Well, keep in mind that the game "lets her advance" so to speak while she's incarcerated. I can't think of a good roleplaying reason for that, but the practical reason is because the game designers didn't want her to be too far behind the rest of your party when you rescue her. You'll have to doublecheck the numbers, but she'll either have 400,000 experience points or 1.3 million experience points when you rescue her, depending on the level of your party. (When I had a party of four Bards plus Aerie, they had about 550,000 experience points each when they rescued Imoen, and Imoen had the maximum number of experience points.) When you edit Imoen, you'll have to decide whether she should still have the same number of experience points that she had when she was kidnapped by the Cowled Wizards or whether she should have whatever amount of experience the game thinks she should have. You have control, so it's up to you.
Back to the issue of party balance. There's a big difference between a small party and a large party. If you have a small party, I think the minimum skill set you need includes one "tank", one Cleric, one Thief, and one Mage. For example, a two-member party comprised of Cleric/Ranger and Imoen (a Thief dualled to a Mage) would fit the bill, although I would beef them up as much as possible. When you play Throne of Bhaal, you'll need another "tank". Sarevok makes a nice addition to the preceding party, especially since all of them are Bhaalspawn. "The Three" against "The Five" makes a nice roleplaying theme. I like those odds.
But anyway, if you have a large party, I think you need to double the requirements, with two exceptions--you only need one Thief, and for Throne of Bhaal, you need about three "tanks", not four.
I thought you were saying that you wanted a full party of six, and I wasn't sure what you wanted your main character to be. First you were talking about a Fighter, then a Berserker dualled to Mage, and now a Berserker dualled to a Thief.
I think it's apparent that you can't always stick with the same character you played in BG1 when you graduate to BG2. BG2 opens up so many more possibilities. For example, I don't think I'd want to play a multi-class Fighter/Thief in BG1 (too much dilution), so I might consider playing a Fighter whom I could dual to a Thief later on in BG2. But dang it, when I start playing BG2, I'd rather have a multi-class Fighter/Thief, expecially if I have installed Throne of Bhaal and can look forward to all of those wonderful High Level Abilities. It's a different game than BG1.
If you have decided that you want Kagain, Yeslick, Kivan, and Imoen in your party, then for the sake of party balance, I think your main character should be some type of spellcaster. Kivan should probably be played as a "tank" if you keep him as a Ranger. Archers are great for the first half of BG2 (they make excellent snipers), but they lose their appeal later on, where the focus shifts to spellcasting and melee instead of sneak attacks. I'm saying that as someone who played an Archer and loved it when I played Shadows of Amn. If you want to make Kivan a Druid, I would question your decision because I don't think it suits him. (I don't think you can make a Ranger/Druid, either.)
I think you need to look at your priorities: do you want to keep playing the same character you played in BG1, or do you want to import all of the NPCs from BG1 who don't make an appearance in BG2, or do you want a well-balanced party? Or, do you want to create a roleplaying character who really appeals to you? They're not necessarily mutually exclusive, but each priority can have an effect on the other. So make up your mind, and we'll go from there.
BTW, I should probably tell you my reasons for doing the Windspear Hills quest and Nalia's Keep and the slavers, etc. The last few times I've played the game, I didn't have a full party of six, and I was very selective about the NPCs I recruited. In Chapter Two, I didn't consider any of the NPCs to be a "permanent party member". My priority was to rescue Imoen, and towards that end, I recruited whatever help I needed to overcome the immediate obstacles. But I also played a "good guy" who didn't ignore the needs of others, especially if they were helping me. That's why I let Nalia and Keldorn join my party, and once they were party members, I looked out for them just as they looked out for me. Although I can't prove it, I suspect that Firkraag was behind the attack on Nalia's Keep. Windspear Hills is also a "Paladin's Quest". So it seemed entirely appropriate to bring Nalia and Keldorn with me. The Unseeing Eye quest was also "Keldorn's quest". And since Nalia hung out at the Copper Coronet, I thought it was appropriate to do the quests associated with the Copper Coronet while Nalia was in my party. Isaea Roenal and the Slave Lords came back to haunt us, as well. I dismissed Keldorn and let him return to his family when we had raised enough money for passge to Spellhold, and I dismissed Nalia because she belongs in Athkatla. In one game, my permanent traveling companions were Imoen, Minsc, and Jaheira. But even in other games that involved other NPCs, once I rescued Imoen, my party was complete. In any case, the roleplaying experience and "meta-gaming" issues fit together quite nicely. IMO, that's the best way to play.