Link to guide?
Might have a look at that myself.
I sign all my coaches as coaches, so they can do youth and senior. Good or bad idea?
I haven't read any guides, but...
I generally sign my coaches to work with first team and the youth and maybe have one youth team specialist.
I will have my assistant manager dabble in training.
Two fitness trainers (one for each type).
Then I have one trainer for each skill that needs training.
They can be 100% devoted to the skills that they're good at, and you don't have a superb attacking coach wasting time on set pieces or fitness.
I don't often have a youth coach, infact 9 times out of 10 I'll find that my main squad coaches can be good enough to coach both squads.
If your a lower league side, having a youth coach may actually be harmful to your normal first team training, simply because the number of coaches you're allowed to have is severely limited.