Ah yes getting spawners is annoying, but the way the gates work, you can chain them easily, I personally haven't tried mirroring the gates but that should work as well so that you can stack these easily. Just have one spawner at the top and let it work its magic by slowly going down the chain.
By more than one input I mean having 3 or 4 different inputs for a single gate. While you can create this functionality with chaining 2-input gates, I think its pretty easy to add inputs to the existing designs. They would take up more space but it would still be way less than chaining gates.
Also, what do you mean by ALU? Nothing is coming to mind for what that means.
Also I think you misread, at the bottom of your post you were talking about outputs while I was talking about inputs. xd
no biggie tho