It depends entirely on how you play regularly, for the most part. My Demon/Pain MM hit 41 last night, so this is based on my experience. I almost always play with my SG, so I'm basically never playing solo, which is something to consider.
I've skipped the three attack powers (Corruption, Lash, Crack Whip) but taken every Pain Dom power as soon as they become available; I'm obviously the healer of the group! This helps a lot with keeping both your team and your demons alive, which is a big boost because, as you said, they are pretty squishy. Leaving out the attacks also means I've been able to skip stamina. Multiple extended fights will still drain my endurance eventually, but I've not had any major problems for not taking it, and not having the attacks frees up both time and endurance for spamming the AoE heal when not doing anything else - this helps keep the demons nicely topped up, which is obviously a good thing. I am quite heavily slotted for End Red in the more costly powers, so that definitely helps too.
Power Pool wise, I've taken Assault and Tactics to boost the destructive power of my demons. You could take Maneuvers too, but I've found that the abundance of healing in Pain Dom is more than sufficient to counter any deficit in defenses under most situations.
I also took Hasten to help keep the tasty buffs and debuffs in Pain Dom up as often as possible, which works out very well for me.
I've started into Chill Mastery for my Ancillary, with a mind to take Hoarfrost, Frozen Armour and Hibernate to cover the obvious holes in the survivability of the character itself, but obviously this choice is much more weighted by your play style than most. So far this has worked out very nicely for me. My demons die only very rarely and are putting out some very nice damage, and the team certainly appreciates the nice buffs and massive amounts of healing I can put out.
Some notes:
My current build is pretty flexible - I actually have two travel powers (Super Speed and Super Jump - go go City Traveller!), but one could easily be dropped for another pool (Maneuvers, should I find that I want it) or an attack. If you're the kind of MM who likes to take all the available attacks, that's obviously a good option!
If you find yourself soloing most of the time, you can also get away with skipping Conduit of Pain and Enforced Morale in lieu of attacks or some more defensive powers like Combat Jumping.
So, that has been my experience with Demons/Pain Dom! Ultimately any choices you make will come down to your own personally play style, so feel free to use or discard this information as you will!
So my DS/pain is pretty tough to be honest. I have the crack whip, for the -res. I picked up both the pet +res aura IOs for my pets, that plus thier base resists, plus the aura from the ember demon, plus World of Pain gives my demons about 70% resists. Besides that, I focused on getting some global recharge so World of Pain would be perma, and so Anguishing cry would be up as often as possible. This also helps the heals come up more often. All in all she plays pretty well.
for /pain, I skipped share pain, because it's overkill generally, slotted the other 2 heals and suppress pain with 5 doctored wounds, to get a recharge bonus, and good heal/recharge. World of pain is 4 slotted with Res/recharge. Don't have painbringer yet, but I expect it will get a doctored wounds as well, and I will be putting it on my Gargoyle.
Thanks for the info. 
I'm up to level 32 now and things have improved quite a bit. My pets feel more survivable (plus, the damage!), I have Provoke to get some tanking on, and WoP smooths things out. I'm finding that /Pain really excels at being flexible. This is easily becoming one of my favourite alts, and I foresee myself rolling more MMs in the future. 
He's sitting at 24 at the moment and I'm at a loss as to what power pool I want to dip into. Do I want Maneuvers to try and beef up my pets' survivability? Do I want Provoke to get a bit of tankerminding in, despite having no mezz protection? Do I want Combat Jumping and possibly Acrobatics later on to get some immob+hold protection? All I have so far is Fitness since I seem to blow through End on teams. I've skipped Lash+Corruption and Conduit of Pain. I might consider dropping Enforced Morale, but being able to clear/prevent status effects is decent so far.
Right now DS/Pain isn't really lending itself to a particular playstyle. The pets are kind of squishy and I don't make the greatest bodyguard tank. So yeah, I could really use some guidance.