Again a globe valve is the valve designed for flow regulation neither ball or gates areFor the OP.
Your 3/4 inch drain for a 55 DT is fine. I use a 1 inch on my 180 and it is silent, safe and even with three 90 degree elbows it flows just fine.
In a herbie overflow it is desirable to have the primary (full syphon) drain at least 6 inches below the emergency drain. Don't sweat it if you can't pull this off. It just makes that initial tank start up quicker and quieter because the primary is able to obtain a full syphon faster.
You can use whatever kind of valve you want for the main drain. However, ball valves are not designed for flow regulation, whereas gate valves are. My personal recommendation is that you buy the best gate valve you can afford.