well, it seems you've addressed the main thing I would have said, which was 'drop the kans'. there are too many large blasts that'll just one-shot the whole squadron without batting an eyelid, but as I said, you've already done that.
nobz. I'd just say, plain & simply, don't. they're overpriced, unfortunately, and will die to any pieplates that anyone slings their way.
3 meganobz in a trukk, it'll work wonders as long as you can get it to move once. it'll work as a distraction, maybe even take 2 units shots away from the main army. If you want them to be a tad more effective, I'd give them kombi-rokkits. skorchas are very powerful and will almost invariably give you a very difficult charge to make after you fire them. if you could overwatch they'd be awesome, but I think being able to crack open a transport with rokkits then get locked in combat with the unit inside is far better than charging a vehicle, showing the opponent how easily they open them up and having them disappear in the next shooting phase. I'd also heartily recommend a boarding plank for the trukk, so the MANs can open up vehicles without getting out. for 5 points, it makes the opponent commit 2 units to deal with 3 models in a flimsy box, which is always good!
just noticed the nobs are bikernobs - I'd still say no to them. there'll be Strength D and S10 all over the place, they'll not survive.
I'll give you the advice I give most people who try and run orky 'deathstars' - don't. in this case I'm referring to the fact that you were thinking of putting the megaboss with the meganobs, then throwing them forward to kill things. Personally, I have much more success by dropping 2 boys from a wagon squad and sticking the boss in there. so you can't overwatch, ah well, but it means that if;
A) the trukk is destroyed turn 1 or
B the meganobs are targeted for complete destruction
then the warboss will still be somewhere else. fact is if the 2 big hitters of your army are running around with a big "neon sign" unit shouting "kill us before we kill you!" then that's exactly what will happen, and you'll be left with a large portion of your army killed off by a small amount of theirs.
with regards to an escort for the bikerboss, I'd be interested in whether you've got any stormboys. people discount them because they're a bit fragile, but with 20 of them escorting the bikerboss, suddenly you'll find it a lot easier to make the tenuous charges, thanks to an extra
D6" every movement phase. as long as you string the unit out so he can keep up, they'll really boost him more than +1
WS would.
onto the space marine section, we've got some things to sort out here...
5 marines can take either a special weapon (meltagun)
or a heavy weapon (multimelta), you need 10 models to take both.
you'll also find that heavy flamers can only be taken by legion of he damned or sternguard. otherwise devastators in drop pods with 4 heavy flamers would be the flavour of the month.
you're also exceeding the number of heavy support slots an allied detachment can take, you'd need a second
HQ in there to double up their force-org.
what was the plan with the flamer unit anyway? they've no way of getting to the enemy safely... were they planning on manning the quad gun? if so why the flamers?
If you have one and can find the points, might I suggest a skyshield for the lootas? it'll raise them up and give them a better view over low obstacles, it'll give them saves against everything except D weapons and it'll give your units something to hide under if there's a lot of barrage floating about. it's a drop-pod landing zone, a firing platform, a force field and an umbrella all in one! as you're onto a dual force org, you can also take the aegis to give anyone underneath cover too!
as for my personal feelings, there's no need to bring orks with allies. I find too often the core of an allied detatchment shows up the weakness of the elite army that gets the job done - the cost. just to get a unit of troops to take out a vehicle and a
HQ to make them available cost you 5 twin-linked rokkit deffkoptas in outflank or scout-moving. the second unit of troops cost you a decent unit of gretchin to hold an objective. the two thunderfire cannons cost you a pair of battlewagons to keep your lootas safe.
It's unfortunate you only have 3 trukks, as I'd say they're the tool to get the job done if you're expecting people to bring expensive superheavies. they'l never get their points back against 12-18 trukks!
also, no
KFF's? there'll be a lot of normal firepower out there, and whilst S

will be there too, most of the shots aimed at the battlewagons will be demolisher cannons, lascannons, mssile launchers - you don't want to go out there naked.