Does it matter on which airplane you build float hours from an insurance point of view? What about a "Fisher Super Koala on Floats" registered ultralight? Would it be an option to build float time?

I don't think an insurance company would go for it. They like time on type.

How much time are you looking at building? They like time on type, But if you are building up a lot of time on it that may help you out a fair bit. if it's only 10 hour or so hours. Then pay a little more and get it on something like a 180.

Or pay less and get it on a 172

Even if it would help with insurance, I can't see it help you getting a job. If I were an employer and saw 200 hours of ultralight float time, I would see 200 hours of float time that didn't include the challenging parts of float flying: big water, big loads, external loads, etc, which a normal 200 float hour pilot should have at least a little exposure to. I'm sure ultralight float flying presents it own challenges, but not to the same degree as some of the more intense float jobs out there.
Also, would ultralight float time even "count"? I don't believe multi-engine ultralight time counts towards multi PIC time does it? Or are all those chasing MPIC time missing out on a real cheap way to get it?