It'd work, but you'll need pre's if you want do 16 xlr's. If you're relying on the mackie, then you'll still only have 8 outs. But things like bass, keyboards, etc could be DI'ed and plugged directly into the hd24, unless youre dead set on mic'ing everything. How much stuff do you have with direct line outs?
I'd maybe think about selling the mixer and getting some pres. If youre going the hd24 route youll be doing all your mixing in that device anyway, using a 1604 for just a few pres is overkill, and the mackie pres arent that great anyway! If you got a fair price, I think you could get 16 (or 24) better pres with the $$.