Its not the number of models. Its Nana's AI that's the problem. If Nana doesn't have the same AI as in the past, then Ice Climbers would work totally different. Pikmin just follow Olimar around, they don't act on their own. It was specifically stated to be Nana's AI that was the problem right from the start, so I'm not sure why this debate is still being had.
As I've stated, the 3DS can handle 6 Fighters at once smoothly at 60 FPS with the Mii Fighters, in addition to a single assist trophy (some assist trophies have basic AI).
First of all the Ice Climbers have less polygons compared to other fighters.
Now if we assume both Popo and Nana run at 60 fps each, the 3DS could easily have 6 of them on screen at once. However, we know Nana would be limited to 30 FPS. Nana's AI is more complex, less complex than a level 9 fighter (I think shes level 3? I'd need to look into that) but more complex than an assist trophy.
So lets say an assist trophy uses 50% AI complexity, compared to a 100% AI fighter (at level 9)
With 6 fighters and one AT on screen at once, that means you would have 650% fighter complexity.
Since Nana is not as complex to a fighter, lets say she has at least 75% AI complexity. Thus 4 sets of level 9 AI controlled Ice Climbers on screen would be about 700% AI complexity (400 for Popo, 300 for Nana). 750% with one AT on screen.
Now keep in mind this is only if we select 4
AI CONTROLLED Ice Climbers at once. If you have 4 human players, that would only have 400 AI Complexity on screen at once. Along with one AT so up to 450%
So we know the 3DS can handle up to 20 characters on screen at a minimum of 30 FPS (4 in 60)
And the 3DS can handle at least 650% of AI complexity. I estimate the Ice Climbers too take around 700%-750% with 8 AIs on screen.
So it seems the AI is just barely pushing the 3DS. Perhaps rather than limiting human players to 4 ICs, its possible to limit the computer from selecting mutltiple ICs. This could be a very great fix and compromise.