Once the sound gets to the Focusrite unit, it'll still be running through a preamp on each channel. The preamps on the Focusrite units are very good, and will carry the signal through cleanly, but if you load another signal onto one/any of them, that signal will be weaker than those which are running a single signal.
Clear?
Using 5 separate signals into 4 preamps, two of those signals will be weaker than the others, and you'd have to do some adjusting, but still won't get the clean output that using the 18i20 would produce with what you have. Everything will be clean and even in the end mix using the 18i20, instead of 3 good signals and 2 weaker ones using the 18i8.
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