Can you share the brushes? I will take a look and see what I can do to help you.

For now, to make a brush, get the image you want to be the brush, open it in GIMP, and go through this steps:
(I will assume you want to use the brush as it is, with it's own colors)
1) Go to Image > Mode > RGB (If it akready is RGB, just skip this step)
2) Make any changes that you want on the brush
3) In new versions of GIMP (I am using 2.8.10), go to File > Export As, and save your file as .gbr (GiMP Brush) on the folder Users/<Your User>/.gimp-2.8/brushes
4) Close and open GIMP again, and it is done, your brush might be there.

You can make brushes that use the foreground color, just set the mode to Grayscale (Image > Mode > Grayscale), but doing so will remove the brush colors, with black being opaque and white will be totally transparent.

I hope this can help you a little.