1. Yes, that will work. I would only stick to tier1 and tier2 (soldiers and spec ops), as it doesnt stack you only have to be sure it proc as much as possible and focus pure dmg on your commando.
2. No, doesnt matter, any lvl will do.
3. Yes, it does. the Proc of heel -res doesnt with itself (asume all 3 tier1 pets proc it).
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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To Clarify....
2. No, doesnt matter, any lvl will do.
Actually it does matter but not in the sense that you mean.
The bonus will not increase or decrease with level but if you have a level 50 IO the bonus sets + procs will not work if you exemp lower than that lvl.
For example, if you enter Warburg or start a StrikeForce that runs at lvl 30, you will loose the bonus from any IO's above that lvl.
The % will stay the same, but the SetBonus and Proc will no longer work.
Consciousness: that annoying time between naps.
thanks for the answers! 
Making a merc / trap mastermind (just for fun) and i have three questions:
1. Can i slot soldiers with 2 damage io's, 2 accuracy io's, achilles heel -res, Touch of Lady Grey: Chance for Negative Damage and get all the bonuses? Or doesn't it work like that?
2. Does it matter what lvl achilles and lady grey enhancements are? Or do they give the same bonus whether they are 17 or 50?
3. Does the mortar -res stack with achilles heel -res?
Thankful for any answers!