Miquon looks neat, but I would not supplement MUS with Miquon. This is because the blocks are too similar while being too different. It would become too confusing.
For MUS, I would add in, if you really want to add something in, Singapore Math Challenging Word Problems. It might go by a different name now. But, since scope and sequence are different, you will want to test your child before adding it in. I would say wait until you hit Gamma in MUS before adding it in and then try level 2. Level 2 in Singapore Math is when they introduce multiplication. Challenging Word Problems actually gives higher level problems so if you purchase level 2, it will actually be working on level 3. This could be complicated too as level 2 really is more of US grade 3 so if it works on the level 3, it might be equal to US grade 4. But I think you could get level 2 when you get to Gamma and try to see what you can do with it.
I hope I have not confused you!