I have used combinations of HEPES buffer and uranyl acetate many times. I don't know if there is an upper concentration, but if you wash the grid before applying the uranyl acetate most of the HEPES will be washed off anyway.

pH is more likely to be a problem, since uranyl acetate is acidic (pH ~4 or below) and unstable above pH ~5 or there about. Since uranyl acetate is also a very fast acting fixative, it doesn't matter to most samples that you acidify your sample while staining.

Thanks Holger!

Holger did mention washing the grid before applying uranyl acetate. You could use the following procedure: float a piece of carbon support film ( about 3 x 4 mm; made by carbon evaporation on a clean mica sheet) on your sample preparation; after appropriate time (needed for attachment of your sample particles to the support film), lift off (from below) the support film (with the sheet of mica from which the carbon film had been floated off) with the adhering sample particles and transfer it to a washing solution (could simply be water), wait a few seconds to dilute the original buffer of the sample, lift it off again with the mica sheet, remove most of the hanging drop with filter paper, transfer it to the surface of your negative staining solution,  lift it off again after 10 to 20  seconds from below with a copper grid that has been treated before with a drop of alkohol, and blot the grid dry  with filter paper from the side of the grid. Such a lifting off from below has  the advantage that the bars of the grid, even after blotting nearly  dry, keep some negative stain close to the bars, resulting in a gradient of negative stain  from the bar towards the center of the field. Often such a gradient allows to select a site on the grid with optimal contrast of the sample particles.