Unless you're going to be running a lot of toggles - or using a lot of attacks - you can safely avoid getting Stamina. Likewise, it's not worth investing in the defense pools if you ask me; usually when you get into direct combat, you tend to be dead anyway.
Assault and Tactics are very useful; slot three ToHit Buffs in it, and you can get away with one Acc Enhancement in your second and third tier units.
The medicine pool would be a wise choice; Aid Other is invaluable and once slotted can easily bring a Hench back from the brink of deat-....defeat.
Finally....Serum. Don't ever get this or I will slap you. Recharge is way too long for the benefit it gives.
Assault+Tactics are always a good bet. Personally I am getting stamina because I want to be able to do 2 things: Use Group fly and all my toggles at the same time and take stuff from the patron power pool to use so I think I'll need it but you may not be building that way.
I recently hit level 40 with my Merc/Traps Mastermind, and my current power pool sets are Leadership (Assault and Tactics), Medicine (Aid Other), and Fitness (Swift, Health and Stamina.)
Yup, thats right no travel power. Sometimes it means a run to places, but if you are sensible you won't be in danger. (Of course, a team-mate with Recall Friend when you have one is nice 
)
For the odd hop, I have had Goldbricker the Skyraider Jetpack (Goldbricker has been used up, but I still have around 50 minutes worth of fuel on the Skyraiders.) Also the Stealth/Combat Invis temporary powers from Bloody Bay and Siren's have helped avoid an occasional enemy.
With I7, I will switch Swift for Hurdle (to help jump over the odd obstacle). Presence may take the place of the Medicine pool - on Test, Challenge and Provoke with the upcoming Bodyguard ability has proven quite a successful combination.
I wouldn't recommend either the Serum or Detonator powers, from Mercenary or Traps powersets, from my experience of them so far.
The Intrepid's / The Institution - Union Server Hero / Villain Supergroups
Thanks guys/gals.
I knew I'd roll one and THEN i7 would hit..... 
As it happens, I popped Serum on my MM in test, used it..... never noticed a damn thing and days later it was still recharging. I agree whole heartedly, steer clear of this like the plague.
Might dabble in the Presence pool then.
Decided, I can live without Stamina with Merc/Traps, they can fight while I rest.
But what happens when my patron pool opens......
Cheers again & if you think of anything else, McCabe will be happy to learn from it.
"Like War_Hero said, leave it as it is, it toughens up the softies." - Naz Nomad
Union Heroes
War Borg - Leader of The Paragon Knights
Union Villains
McCabe - Co-leader of The Rogue Knights
Defiant - Supreme Hero
For power pool advice the ones I would take are:
Medecin - at least aid other and aid self, helps keep henches an dyou alive. If you want rez take it, I have it cause I didnt want anything else.
Leadership - I like this power, might be even more usefull for thugs to offset the res most mobs have to leathal damage.
Travel power - I went fly for group fly to keep all henches with me. Any works I just find fly best for my MM.
Power wise from the primary avoid M30 grenade it is a worse choice than serum. Also avoid serum. Burst is ok but if you want a personal attack go slug.
From the secondary the ones I am not taking are: triage beacon (might change if at later levels I have space), trip mine and possibly detonator (may only take this for concept). Seeker drones can be left out if so desired but I am going to try out how effective they are.
Trip mines are great - laying a big bunch and then watching a Boss of +3 levels come around a corner and get blown sky high makes them worth while
And if you happen to be teaming with someone who has the Wormhole power... KABOOM! Can take out whole mob groups in one shot!
With 2 recharge reductions slotted, you can lay around 15-16 mines before the first expires. Thats a lot of bang for your buck.
The Intrepid's / The Institution - Union Server Hero / Villain Supergroups
Just rolled a soldier MM with his faithful mercs, he's got traps as secondary.
But what sets should I be choosing for him?
Naturally for me, a travel powerset will be thrown in by lvl 14.
Does an MM require Fitness? Or should I let my henchmen do all the work, only helping where need be?
What others?
I've never had Fighting but thought the added defences may be handy.
Leadership was another consideration, buff my men up. I'm assuming vengeance can't be used on a 'henchie'?
Any tips handy, as doing the respec with an MM is not appealing!
"oh, you're ruddy henchmen have stuffed it again!"
Cheers in advance
"Like War_Hero said, leave it as it is, it toughens up the softies." - Naz Nomad
Union Heroes
War Borg - Leader of The Paragon Knights
Union Villains
McCabe - Co-leader of The Rogue Knights
Defiant - Supreme Hero