First, blacknet is right on the money. You can never carry too much light with you. If you have too much, you can either manually adjust the output or let TTL do the work. If you don't have enough, you are just plain out of luck.
Now, in terms of what strobe to add. One school of thought is to have two of the same strobes. Some say this creates a flat picture, but I think that is hogwash. If you want to vary the light, just adjust the strobe angle. Two of the same strobe output eliminates the problem of unbalanced light for certain shots, especially in wider angle work, where you would more likely be using manual strobe control.
The second school of thought is go with a larger unit. More light is better, right? But it does present some complications. Unless you are purposely trying to highlight an object on one side or another, you've got to set your strobes at different outputs. If you are trying to capture an object in close focus wide angle and the subject is on one side of your frame, you face a daunting task of trying to get the correct strobe output on both strobes. Often, you will compose a shot where you wish the larger were on the other side.
I would go with another equal strobe.
I would also stick with another Sea & Sea. If you go with slave mode, it doesn't matter as much, but mixing strobe brands complicates dual sync cords and arm attachments.
I own three Ikelite SS200s and one Nikonos SB-104. I shoot two SS-200s on my housed system and use the other two as single strobes on Nikonos V's. I'm a big Ikelite fan, but again, I think you would be better served with another Sea & Sea. Remember, keep equipment and setups simple, let your task loading be strictly composing the shot.