Your first requirement -- float all day w/o floatant - almost requires that you use cork or foam as a construction material. If you don't like these then I would suggest ram's wool treated with "Hydrophobic Fumed Silica Gel". I think you have my book - see Chap 18 on ram's wool. The Hydrophobic fumed Silica Gel Is actually a super fine white powder that will penetrate the wool and repel water. Finished flies will be lighter than cork and many foams.
For patterns - ants and beetles are a good start.
As far as rubber legs there are some very fine ones available that should work on even very small flies. Also Doug Swisher makes a series of different dubbings containing super fine "wigglys", (his term for the strands). ( He also sells packets of the super fine strands by themselves not mixed with other dubbing materials in several different packaging options.
I don't think Swisher uses rubber strands. They look more like Spandex, which as you know is sold to fly tiers as "Stretch Floss". This would be a very good substitute for rubber legs as it can be split as fine as you desire.
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