Is there an equation to convert mass to volume assuming a negligible gravitational effect. So say 1 micro gram occupies a particular volume.
The relationship between mass and volume is called the Density (look it up). Density varies from substance to substance. Gravity doesn't come into it unless you are dealing with a substance that is compressible under its own weight (say air).
But a Planck volume is not 1 m3, it is (1 Planck length)3, or about 4 × 10-105 m3.... I was thinking I could scale them up somehow to say make 1 Plank volume equal to 1 cubic metre for convenience and then derive the other units from that.
I think the idea was to divide it by 4x10-105 to make it easier to work with.
You aren't really explaining what you are trying to do, so I have no idea what the issue is. Perhaps you need to be working with the equations without plugging in the values?
Yes, if you don't want to deal with ugly values, working with equations can avoid that.
And yes, you can certainly find tables of densities online. However, be aware that density is a macro phenomenon that depends on the arrangement of the atoms. There is not one single density for an element. This probably means that what you are seeking to do isn't possible.