>>>>>HOT Magnum; to which I might try to add a Eheim surface extractor.
What concerns me is that it is rated at 250 gal/hr (though I think
that is through empty box) and I might end up with too much water
movement. I also have in mind using the Bio-Wheel adaptor (a tee) to
split the outflow in two, one just above the bottom and the other one
at the opposite end of the tank just below the surface, both slightly
angled towards the front. Also, does anybody have any experience
injecting CO2 in a HOT Magnum ? I'm concerned about the vapor lock
possibility, like in the Fluvals.<<<<<

I have an HOT Magnum on a 20 long, and I have a DIY CO2 injected into the intake. My setup sounds very similar to what you are contemplating. I have had this up and running for about 4 weeks without any problems. The Magnum does generate quite a water flow, but I don't think this has hurt anything, and may even be beneficial. As for any vapor-lock problems, they don't exist. Occasionally you can hear the bubbling of the CO2 into the filter, but it is hardly noticeable.