OK, First question:
I don't use head cement, havem't in years
Second question:
dry flies & nymphs=down eye
spinners=up eye
any fly smaller then 26 I use a straight eye
Third question:
using over sized hackle will cause your fly to stand on end or not float properly, also I blocks the hook point.
lastly:
don't get hung up with exact patterns, tie flies with what you have to match the flies hatching, often the ugliest flies catch the most fish.
The man who coined the phrase "Money can't buy happiness", never bought himself a good fly rod!