Derailleur hanger / dropout replacement : A checklist

Example of a hanger

Bicycle Derailleur Hanger replacement
Replacing a hanger can be a nightmare.
What are the 'rules' when replacing one ?

Here is my 'naive' list for when having to replace your hanger.
My knowledge : 3/10

Is there a universal/generic hanger ?	No. Your hanger will likely be specific to your frame
do the bike makers make spare hangers ?	It seems most don't. CNC machine companies sell copies on Ebay/Amazon etc.
How do I identify my hanger ?	If you know the bike make and model and year. But in the end it seems theVISUAL comparison is the most important bit. It has to look very like a copy on a CNC list of hangers
Are the generic hangers in shops any use ?	Probably only as a 'get me home' hanger, if that. It probably will fit badlyif at all
How many hanger types are there ?	100+ ?
Where should I buy ?	Probably Ebay from a CNC comapny. Check their photo list or send them an email. You could send them a photo of your hanger to help
Try the bike maker ?	You could try
Hanger bolt	If you still have your old hanger bolt you can use that. If not, you'd be best buying the bolt too. This will be an option by Ebay sellers
Price	Expect maybe around £7/$15 without bolt ? Add another £1/$1 for bolt

Shimano Rear Derailleur cage sizes SS GS SGS

Shimano rear derailleur cage sizes (reference)

Short - SS

Medium - GS

Long - SGS

My (naive) understanding.

When buying a rear derailleur (Shimano code RD)
Also keep in mind the cage length
In general short cage (SS) will have a short looking cage and long a long lookingcage
My understanding is that longer cages can deal with bigger teeth gears (front and/or back)

Examples :
A short cage (SS) probably won't cope with a 34 teeth big gear on a rear cassette. Maybe best for 26t rear max (?)
A long cage will likely cope with a 34 teeth rear cassette

Keep in mind to take into account the size of your front gears
It's about how much chain the rear cage can deal with from both the front and rear gear sizes. 

Road bikes often seem to have a 'smaller' 1st rear gear (e.g. 26 teeth) and SS cages are often for them (probably the point is that SS will be quicker for changing gear)
MTB's often have a 32 or 34 teeth rear 1st gear (big looking) and will probably need a GS or SGS (can deal with bigger teeth on rear or front)

My current status on this subject : pretty unsure but I have a 34t rear cassette so going for SGS to be safe. Awaiting more wisdom.