I have never used the HIG because it did not exist when I first started homeschooling. Then, initially, they said they wrote it to help those who complained that it was too difficult to teach math and wanted a more scripted program.
If your child already mastered the material and you are just going over the textbook for review, then I would not worry about the textbook. However, you are talking also about supplementing with Math Mammoth. If you need to supplement, you should first use the workbook intended to go with the program. Then if that is not enough, then add something else in.
I do want to throw serious caution out to you. Others warned me when I started homeschooling, and I did not listen. My child quickly headed for burn out, and we had to change. I know all the programs look so great. And when you cannot decide between two programs, it seems so tempting to cut one or both short and just use both. This does NOT help your child. This will burn out your child, plus, they never truley get the full use of either program. I tried Singapore Math textbooks with Horizon math workbooks when I first homeschooled. Big mistake. In the end, now, after many years of homeschooling, I have learned that while you may find many wonderful looking and tempting programs out there, you just need to settle on one and go with it. If you do not, you will burn yourself and your children out. They will end up hating the subjects, you may give up homeschooling, or you will change to adapt.
I have heard here many times over that Math Mammoth is supposed to be great. If you want then, just use that. Seriously, if you try it for a year and don't like it, you can always switch then. Not like you are committing for the rest of forever, even if it feels like that. Trust me, I know what it is like to be very curious about multiple things or wanting to try multiple things. But the absolutely best thing you can do is decide which you like best and go with it. Do the Singapore Math program with the workbook, or the math mammoth, but don't try to piece them together and skip a bit in each.