Well personally I see the main advantage being the exact opposite - you encourage overkill. If your opponent wants to jump that assault squad on your pathfinders, or unleash that squad of 20-odd gaunts on them, they only kill 4 finders, whilst the other 4 are off prattling about somewhere else. This instead of losing 8 guys as you would in a single squad.
It's not simple min maxing because you aren't taking the pathfinders purely for the fish. The smaller pathfinder squads are actually better units - for the flexibility, and resilience reasons mentioned - than the single larger squad. The thing that prohibited multiple small pathfinder units before was the fact that the fish was so expensive. But an SMS fish being a more attractive prospect opens up this possibility for me.
Yes you're hard pushed for Fast attack slots if you want vespids and pirahnas. But I'm certainly considering 2 pathfinder squads of 4 finders + SMS 'fish and a squad of vespids and simply doing without the pirahnas.
The one point that does concern me is how much SMS fish and pirahna's are in direct competition with each other - pirahna's are scoring units and fast skimmers, whilst fish are non-scoring, but have greater firepower per model (though not per point) but also have tougher front and side armour.
It may turn out that, in general, I'm better off putting points into pirahna's rather than SMS fish, and so actually this pathfinder idea (and perhaps even the "lots of SMS fish in your army
" idea) doesn't cut it. We shall have to see - and have a fun time finding out