UA plugs need the UA processor(s) to work. The more processors you have, the more plugs you can run at once. I bought the Apollo Quad and it's fine for my needs. Two chips might be tough for full-blown ensemble mixing.
Mac with the Apollo Quad w/ TB card here, works great. You would be wise to check for Windows results via TB here, there may be some issues for folks with certain TB cards or mom boards. Way out of my experience set. There are many existing posts which you should search, rather than waiting for others to help you.
Take the Avalon via TRS 1/4" to bypass the Apollo preamps. However, if you took XLR from the Avalon into the Apollo XLR ins, most likely you'd be the only one on the planet to ever know.
DO examine your monitoring and cue mix needs before buying. Some here report difficulty creating and routing all the headphone mixes they need. For me it's fine, but I usually rent a big room with larger sessions.
DO realize that when you utilize higher input counts, the bandwidth drops. For instance, you cannot record 16 channels at 96 or 192 (unless I'm misinformed) so there is trade-off between bandwidth and number of inputs. Good luck.