How to Calculate your Shadows Skill Levels
Shadows' skills are 1/2(your Assassin's level in that skill including +skills) rounded down PLUS 1/3(level of shadow at time of casting including +skills), rounded down. So, if you had a lvl 24 MB and slvl 9 SM, your SM would cast an slvl 15 MB
Shadows Get Synergies
Shadows do not appear to have a synergy cap of 20 points, as per other classes, but continue to gain synergy bonuses even after slvl 20, assuming the Assassin has +skills that will boost that level above 20(potentially allowing the Shadow to do more damage with a certain skill than the Assassin herself, or at least matching it). However, to gain the synergy bonuses of a certain skill, it must choose to use that skill (for example, a Shadow would need to use Fists of Fire before getting Fists of Fire's synergy bonuses on PS's Meteor. The same principle works with Lightning Traps, for example). Note: Once that particular Shadow dies (or you re-cast it), it has to "re-learn" all its synergies.
While it's possible to get a Shadow to do more damage with a Lightning Trap than yourself, it often becomes a trying exercise in order to get your Shadow to use the skill you want it to (given how fickle the Shadow AI is). It's easier to control which skill a Shadow Warriors uses, but it gets difficult keeping even a high level SWarrior alive in Hell.
Your Shadows Equipment
SWarriors get all their bonuses at level one
Magical Blade Talon on their left hand
Magical Cestus on their right hand
Magical version of the your own armor's base item (e.g. if you wore a Spirit Shroud, they would spawn with a magical version of Ghost Armor)
Magical version of your own helm's base item
At level 1, SMs spawn with
Superior Suwayyah/War Fist/Scissors Suwayyah on their right hand
Superior Battle Cestus/Runic Talon on their left hand
Superior version of your own armor's base item
At level 5, SMs spawn with
Superior Bramble Mitts/Vambraces/Orge Gauntlets
At level 6, SMs spawn with
Magical version of your own armor's base item
Magical version of your own helm's base item
Magical Bramble Mitts/Vambraces/Orge Gauntlets
Magical Suwayyah/War Fist/Scissors Suwayyah on their right hand
Magical Battle Cestus/Runic Talon on their left hand
At level 9, SMs spawn with
Magical right ring
At level 11, SMs spawn with
Rare version of your own armor's base item
Rare version of your own helm's base item
Rare right ring
Rare Bramble Mitts/Vambraces/Orge Gauntlets
Rare Suwayyah/War Fist/Scissors Suwayyah on their right hand
Rare Battle Cestus/Runic Talon on their left hand
At level 13, SMs spawn with
Rare left ring
At level 17, SMs spawn with
Rare amulet
So, all in all, a maxed out (item-wise) SM would spawn with
Rare version of your own armor's base item
Rare version of your own helm's base item
Rare left ring
Rare right ring
Rare amulet
Rare Bramble Mitts/Vambraces/Orge Gauntlets
Rare Suwayyah/War Fist/Scissors Suwayyah on their right hand
Rare Battle Cestus/Runic Talon on their left hand
The level of equipment a Shadow can spawn with is 24 + 3/level of Shadow at time of casting.
Note: Both SMs and SWarriors do not spawn with Belts, Shields and Boots at any level.