The only advice I can offer is make damn sure you measure before purchasing anything. Typically in-ceiling speakers have to be at least 1 inch away from joists in the ceiling.
The builders put a standard j-box in the ceiling of my house. I bought some Polk in-ceiling but if I were to mount them the grill would not cover the rectangular opening from the j-box and would require a lot of drywall work for the ceiling and re-painting. Too much hassle at this point so I abandoned the idea.
If you have no such restrictions, it is not hard to install in-ceiling speakers but I'd suggest actually going into the attic and making sure you can mount them where you want with minimum hassle. I don't understand 'over the high hat ceiling lights' though. They will have to be separate and apart from anything else that is already in the ceiling - they cannot share space with anything else.