I tried to mix 2 fluids (red and blue) to create the feedback connection in the fluid, barely worked
Did you check the fluid container's "Links" fields? Maybe the fluids aren't connected yet? Drag "Fluid" to "coll" abd vice versa to link the fluids. You should also check the "Links" of all daemons and collider tags involved to see if they're properly connected.
Then you're talking about many crashes. Is it better now with the 0040 version or do you still have these problems? Do you perhaps use the GPU with multi-fluid simulations? If yes, could you please try to simulate via the CPU only? I've also experienced lots of crashes with the GPU and different materials like water & foam, sand & water, etc. Since I've disabled the GPU for these kinds of simulations everything's much more stable. TFD seems to play a role as well, but here we only had trouble with multi-GPU systems and this is should be fixed with the 0040 patch.