Hi barber,
I will go along with the people who say try a detector to be sure. Any of the better nugget hunting VLF's will work but may not detect the wheat grain size nuggets to the depth you mentioned. To be honest, a lot depends upon the nuggets and their characteristics.
As for a PI, absolutely make sure to test them. PI's do not like small gold nearly as well as a good nugget hunting VLF unit, contrary to all the propaganda out there.
As a perfect example, I have a few nuggets commonly referred to as "invisible nuggets" I use for testing that even the newest ML model can't detect even though they are several almost a half gram in size and I have had several of the better known GP owners try. Some Whites TDI's will get a whisper if these nuggets are rubbed on the coil, but that is all. These same nuggets generate a nice strong signal on a VLF, but can only be detected to maybe 3 inches or so.
The point is, there is much more to the nuggets than just the size that makes a nugget detectable with a PI. Very coarse small nuggets have a tendency to be even harder to detect than very solid small nuggets.
The PI will work much smoother around hotrocks, but if it can't detect the gold, then it will be too smooth. So, once again, make sure to take time to test different detectors to see what works and what doesn't for you.
Now, I have a TDI, a ML SD2200, a GS 5, and a specially designed light weight low powered PI that can detect much smaller gold better than any of the other above mentioned PI's and I wouldn't say for certain that low powered PI would work fine on the gold you mentioned without trying it.
As for the nugget hunting VLF's, I have a Fisher GB 2, a Whites GM 4, a Tesoro LST, a Treasure Baron Gold trax, a ML XT18000, an older Lobo, and another older ML VLF. Of those detectors, I give the edge to the GB 2 or the GM4, with the XT18000 a close second. The XT18000 does have a little better auto ground balance in my opinion and is a little easier for a novice to use in my opinion. The LST is good also, but not quite as good on the real small stuff.
Once again, people's preferences are not the same, so it really is a good idea to test any and all models you think you might want to use to see if it really fits your needs. What one person might like another may not and visa versa.
Regardless of the one you end up with, I strongly suspect you will not be able to detect most of the smaller wheat grain size nuggets more than a few inches deep at best.
Reg