Zeta potential is a property of the interface between the dispersed material (e.g., dissolved guar gum polyelectrolyte) and the dispersion medium. The medium you choose will, in part, determine the zeta potential that is developed.

Likewise, the extent of charge development on a polyelectrolyte molecule can affect the overall balance of forces, causing the molecule to unravel or coil up - hence the size may also change with the choice of dispersion medium.

By far the most traditional method (for many decades), is to prepare solutions of your material in solutions of a simple electrolyte (e.g., potassium chloride) at a range of concentrations (0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50 mmol/dm3) and, for each concentration, a range of pH values. If resources are limited, start at 10 mmol/dm3 [KCl(aq)] and look at, say, 5 pH values between 3 and 10. Don't use buffers to try to control pH, just adjust with the corresponding acid and base for KCl (i.e., HCl and KOH).

The Zetasizer requires you select a mathematical model for the estimation of zeta potential from the experimentally measured quantity (electrophoretic velocity). Stick with the Smoluchowski model. There's no point trying to do the more advanced calculations. They assume your particles are hard spheres with uniformly distributed surface charge. Polyelectrolytes in soluion are porous and have spatially-fixed charges. The reported zeta potentials using the simpler calculation may not be accurate (compared to the theoretical value) but the trends you see across pH etc will be valid.

Note, even though you will be forming a colloid, this is likely a situation where it is correct to call it a solution. This is because the guar gum polyelectrolyte molecules are soluble and you form a hydrophilic colloid, rather than a more typical hydrophobic colloid.

See https://drgudinho.files.wordpress.com/2017/04/cours-1-2017-guadalajara.pdf for a thorough discussion of polysaccharide solution behavior.