Those are not golden rules, rules are there to be broken!
But those are good gameplay advice.
. Visibility
Enemy bullets should always be visible: use strong colors and shapes, and good contrast with the background.
. Enemies health points
Don’t add too many hard-to-kill enemies. A simple rule could be: the smaller the enemy, the fewer Health Points.
. Don’t be unfair
Some frustrating gameplay elements: unavoidable attacks, huge lasers starting without warning, enemies spawning where the player is likely to be. You can add particle effects or Text triggers to display warnings to the player.
. Hitbox
The player’s hitbox should be clearly indicated and often small. Enemies bullets hitbox could be small.
. Precise movement
A lot of people dislike inertia in shmups: moves should be reliable and precise, and consistent with the game world.
. Tension
Vary the rhythm, add several different enemies at the same time (often, the addition of different waves create complex gameplay for the player), try to mix every element of your gameplay in different combinations during the levels…. Think of encouraging the player to kill and dodge at the same time.
CLASSIC GAMES
Watch shoot ’em up gameplay videos and try to understand what makes their gameplay successful.
Some shoot ’em up classics:
. Crimzon Clover
. Battle Garegga
. DoDonPachi Dai-Ou-Jou
. R-type
. Mushihimesama Futari
. Darius Gaiden
. Radiant Silvergun
. Gradius V
. Strikers 1945
…