Thats genius...The Ladik P-520 is exactly what i was thinking of I would need for my approach. Are the Outputs 6.3mm jacks?

3.5mm, but it's a simple matter to use 3.5mm to 1/4" cables between the Ladik and the mixer.

As for a mixer i am looking for one with built in USB Audio Interface. Don't need onboard effects since I would probably
never use them.

Smart move. Most onboard FX are sort of "cripply" when compared to a serious FX processor...either the quality is sub-par, or the controllability of the FX are poor, or both. My suggestion: an Allen & Heath ZED-18, which would give you plenty of inputs for your modular submix alongside a good amount for other incoming sources. This has 10 mono inputs with inserts (VERY useful for doing FX per channel via outboard processors!), four stereo ins, four AUXes, ample I/O for output and monitoring (and inserts on your stereo main out), no FX, and USB send/return. Might seem a bit big at first, but the fact is that you always want to overspec your mixer...you'll be surprised at how quickly you can go through channels when you've got everything hooked up!