From the primary, you'll want Call Thugs, Equip Thugs, Call Enforcer, Gang War, Call Bruiser, Upgrade Equipment.
From the secondary, you'll probably want everything. Also, Storm Summoning can be an endurance heavy secondary, so I recommend Stamina. I also recommend Hasten and Super Speed due to the fact that Hasten helps a lot with Lightning Storm and Freezing Rain and Thunderclap's recharge. Super Speed stacks with Steamy Mist for full normal stealth.
In the epic sets, I don't remember if any of the AoE immobilize powers have -knockback. Electric Fences might, but I forget. You'll want to decide (assuming at least one of the epics has an AoE immobilize with -knockback) whether you prefer an AoE immob with knockback, so that you can keep things from being knocked back with Tornado/Lightning Storm, OR whether you'd rather have an AoE immob that won't prevent knockback, so that you'll get knockdown when combining the AoE immob with Freezing Rain, Lightning Storm, Tornado, etc.
As for slotting, I'll just throw out a generic build in mids, followed by a fairly cheap set build with the same slotting, then followed by my build which has my own quirky build (2 travel powers for fun, less stuff from the epics).
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Random AT Generation!
"I remember... the Alamo." -- Pee-wee Herman
"Oh don't worry. I always leave things to the last moment." -- The Doctor
"Telescopes are time machines." -- Carl Sagan
Thanks for the advicing Lewis. Let me give some feedback: Of course I'll have the summon primaries, I am lvl 26, or actually 27 
And with this said, I know that Stamina ill be needed; my end isn't all that ma-min-x, but I certainly do experience end issues, when I'm desperately trying to heal my pets with O2 Boost. And about the epic pool I am gonna choose Leviathan Mastery, for concept reasons.
@Global: Difficult One
Playing on European Servers (Union, Defiant)
If a person turns down an idea, he turns down an opportunity to evolve himself
Storm is usually a very end heavy set, but with masterminds they took it up another notch on the crazy scale.
Stamina alone is really hardly noticeable it is so bad. Heavy, heavy end slotting is needed in everything.....from pet summons and upgrades to all of your storm powers. End slotting should be the primary attribute you slot for in any storm mastermind unless you want to just guzzle blues all day long.
Until you get further along and decide just what your personal priorities and style are, that's the best generic advice I can give. You never really solve end issues with a MM Stormie, you just get to the point you chug less blues. 
Hi all,
So, I'm leveling a Thugs/Storm MM atm, and I'd like some advice as to what powers I should choose and how to slot them. I am at lvl 26 atm so I know how the sets work.
I do not have many plans for the sets, don't know if I'll go farming, AV soloing or whatever, just so you know.
Thanks in advance!
@Global: Difficult One
Playing on European Servers (Union, Defiant)
If a person turns down an idea, he turns down an opportunity to evolve himself