AM not enough. You'll need AM, stamina, and conserve power for AVs.
Cinders>choking cloud any time. I'd go for char, em pulse and cinders (+fire cages and flashfire).
Smoke is nice to have but not a must. Fire/rad is really tight so I skipped it on my build
uhm...Cinders > choking cloud?? Really? :/
Isnt choking cloud an holind cloud always active ? (PBAoE toggle)
I thought it was able to hold every foes that come closer, to let hot feet burn them :P
It's a 50% chance to hold. It is quite good I find.
Cinders has a lower accuracy, (75% on hero planner), but choking cloud has 50% chance to hit, which can't be increased. In other words, cinders unslotted will hit 75%, choking cloud will only hit above 50% if you're lucky. Add to this choking cloud's hold is half as long and drains even more endurance from an already draining powerset and I think you'll see why most people take cinders.
Also, choking cloud is mag2 (minions only) as opposed to cinders mag3 (minions and lieutenants).
The devs try to make it so primary powers > secondary powers > power pools.
Chocking cloud doesn't have 50% chance to hit, it has normal accuracy. What it does have is 50% chance to apply the mag2 hold, and 80% chance to apply a mag1 hold. However, it ticks at 5-second intervals, and lasts 7.45 seconds unslotted at L50, which means that you can, on average, perma-hold most minions, and some lieutenants. And this is a power that is PERMANENTLY on, instead of being a long recharge clicky.
Uhm im so confused :/
I didnt know about the sucking MAG on choking cloud but i supposed it use to drain a lot of endurance.
Anyways im still confused about Cinders or choking cloud, because cinders has a good mag but is mono-use and has a looong recharge, plus i already have Aoe hold powers with other controllers so i would liek to take something different; at the same time, chocking cloud is always on and it try to hold foes every 5 seconds but its an endurance drain (expecially if you use hot feet and the 2 radiant debuff too).
So... uhm..... :/
Personally I much prefer Choking Cloud to Cinders, but that's probably due to the way I've built my Fire / Rad.
For either of them to take effect you have to be stood right in the thick of the action, and I'd much rather have a toggle that takes care of holding anything within real close proximity as it leaves me free to focus more on locking down the mobs on the out-skirts of the pack.
That and the fact that it's nice to be able to debuff, Fire Cages + FlashFire a large group, then jump into the middle of them knowing that between Choking Cloud, Hot Feet and my imps there's not going to be too much that is going to have much chance to do anything to me or the team I'm on.
As others have said, Stamina's a must for any vaguely normal Fire/Rad build, along with AM 3-slotted for EndMod...and you'll still run dry if you have most of your toggles up and are reasonably active on the clickies.
Choking Cloud vs Cinders: I found CC very useful when I first picked it up, at 28, but for me it became less and less so as I moved beyond the mid-30s. The problems are that as you get higher there are more and more mezzing mobs, each dropping your toggles every time, and CC does have a hellishly long recharge once it's gone down. For some missions (looking at you, Malta 
) I found I was only getting chance to put it up after fights were over, then it wouldn't stay up for very long at all once the next fight started. Combine that with the fact that everyone's harder at that level, so difficulty settings are higher and mobs have a greater level advantage, and you find that even when up it's not having that great an impact any more.
Cinders was more consistently useful - a bit of a panic power to lock things down sharpish if they were looking iffy, but very useful for all that (Flashfire's more the bread and butter "ready whenever you want" mezz attack though).
Smoke I had in the early levels, respecced out in the low 20s, and didn't retake until 49 when I couldn't find anything better to take. It is a bit situational, and best used as a blinder than for its -Acc (which is feeble) - it can be very useful to throw at nearby groups if you think things might get untidy with your current fight, and as it's auto-hit and non-aggroing there's no downside to using it "just in case".
If you're planning to solo much after you're Imped up you might find Smoke useful there too - after getting it at 49 I now regret I hadn't had it earlier myself. That's because one of the fiddly things with Imps is getting close enough to mobs to drag them into their aggro range, but not so close you pick up aggro yourself and take the alpha. Smoke the mobs first, and you can get as close as you like to set the little darlings on their targets. 
I've just rolled a Fire/Rad, and im not sure about some powers.
First, what do you say about stamina?
AM already gives a good recovery, but Fire/rad set is based on toggles: Hot Feet, Chocking cloud, Radiation infection and Enervating field are all good powers but being toggles, they drain a lot of endurance.
Im planning to get them all, but im not sure if AM will be enough or if i'll need stamina.
What do you say?:P
Hold powers: Fire/Rad can get 4 hold powers!! I obviously have Char, im sure i'll get Chocking cloud and EM pulse.. but what about Cinders?
We know trollers AoE hold powers sucks and they need to be full stlotted to be something decent, i usually take them with other trollers because i think trollers have to get 2 hold powers (well, maybe not Mind and illusion cause they have great fear powers), but does a fire/rad need it? or it becomes helpless?
Smoke: strange power... i think it has his uses but does a Fire/rad need it? /rad already gives a good -acc debuff so i think you're not going to use it as acc debuff but just to reduce perception and go unseen.. is that really helpful, just good or totally helpless?
That's it i guess, any advices is welcome :P