Yes, I think that is a fair amount, I usually do that amount per 5L. If its a fulvic extract similar to that I made myself. I would think the strength would not even nearly be the same. Mine being weaker to some extent. Honestly, I just dissolved whole L tub of Fulvic powder in 2L distilled water. And its effective stuff at that dose.
The liquids are tricky for this reason. But ya, if you worried about the Humic having a decent amount of potassium in it, then rightly so, all humic acids, well the cheap china stuff we get is Leonardite stone (so they say) extracted via chemical potassium extraction. So there is some potassium in it unfortunately. Its okay to have a lot in veg and end of flower, but still too much isn't great. In your case I think the advantage would outweigh the issue it could cause if mis-used.
I think equal parts of each, around 3-5ml each per 5l will do good.