As far as flotation, the differential between two densities is the determining factor. A piece of steel(density = 7.8 g/cm3) will float like a cork in mercury (density = 13.5 g/cm3). The density of air is .00121 g/cm3 while the density of helium is .000167g/cm3. Therefore, air is 7.24 times more dense than helium. Without experimentation, I would support that helium would add some buoyancy, but at the same time helium has a much smaller molecule. Therefore, helium would probablly leak from either PVC, urethane, and hypalon bladders. I would stick to air.
As far as flammability, Helium is classified as an inert noble gas. In other words, it does not chemically combine with other elements by ordinanry means. Flammability is defined as the ability burn (combine quickly with oxygen). A combustion reaction does not take place between oxygen and helium due to helium having a full valence electron structure.
On the other hand, the combustion reaction between oxygen and hydrogen takes place rapidly and verges on explosiveness. To my knowledge, the Hindenburg was filled only with hydrogen.
A science teacher's 2 cents