The dont add fat advice that you have been given is sound but has been given out of context. If you use the traditional shoulder and belly recipe then the belly has the fat already so no you dont need to add any to that. If you are mincing up bits and pieces of lean meat then yes 20 to 30 % fat.
The skin of the sausage acts like a pressure cooker and along with the hot fat inside the sausage cooks the meat. The fat also transports the meat flavours and juices into the rusk which absorbs it and creates a moist, juicy flavoursome sausage.
The secret to a sausage comprises of 5 key elements of meat, fat, flavour enhancer ( salt, herbs ) rusk and casing, which works. These days sadly many people seem to think that the rusk and the fat are just cheap fillers but they are all essential to make a great sausage.
I use 80% lean meat and pinhead rusk, salt, pepper and herbs or a pre mixed seasoning from butchers sundries and only mince once on a coarse setting. By all means play around with your sausage recipe until you get it just how you like it. The bast way is to mix a small amount of your ingredients then make a patty and fry it off and taste it. If its good mix the rest and fill the casings. If its not adjust your ratios. Sausage making is great fun. Enjoy!
heres some I made earlier