Painting Night Goblins is painfully easy, anyway. 90% of the model is black; you just have to pick out the green bitz, highlight it and wash it, highlight the robes with grey; paint the weapon haft brown, the spear tip with silver, wash the spear with devlan mud; paint the shield red, then paint the raised parts of it gold. Wash that with a bit of devlan mud as well, and if you really are feeling creative, you can stipple it with a bit of Hawk Turquoise to represent verdigris on the gold. Base it, and you're done. Paint 'em in batches of 10 and they go FAST.
As for the original questions:
Full Command is a necessity. Musicians help your units rally, which is important when you're Ld5. A Standard Bearer helps with static combat; and a Champion lets you take an enemy character on without losing several goblins (and potentially a rank bonus in the process) by forfeiting the champion.
A Great Shaman does wonders in a Goblin army. Two of 'em might not be so bad to have. With 8th, you can't have duplicate spells, so I wouldn't bring more than a total of six levels of Goblin casters-- a pair of Level 3 Shamen might not be a bad solution there in high points. You can bring a couple Orc shamen at high points if you want more magic. Goblin characters are dirt cheap, so you'll have a lot of points to spend on Goblin Warbosses and Big Bosses, especially at high points levels. You could even reasonably bring a "unit" of mega squig hoppers by bringing four or five Night Goblin Big Bosses on giant cave squigs, and just having 'em romp around together.
Both Trolls and Giants are definitely things to consider. At 11,000 points, you've got lots of points available to spend in Rare choices (up to 25% of 11,000 is 2750-- you can buy a LOT of trolls for that!). You only can bring four copies of a given Rare choice, so you'll end up with four doom divers, four units of Trolls, and four giants if you decide to spend the entirety of your rare allotment.
If you bring Skarsnik along, he'll be your General. I'm not a fan of using named characters, but in an 11,000 point game, I think it's a bit more reasonable for one to come along. You could also bring Grom, if you like-- it's not like they're expensive, and you have 2750 points out of that eleven thousand to spend on Lords.
If you play an 11,000 point game, you're not going to have flankers. You're going to have a big, seething battle line-- a green tide, if you will. With that in mind, you should bring some Wolf Riders along, because they're among the fastest cavalry in the game, and they're cheap enough that you can bring big units of 'em. If you put them into reserve (which you can do in Fantasy in 8th now!) they also can be quite useful as a timely interceptor of enemy cavalry, which gives you time to reposition your big infantry units to face the most immediate threat.
With 11,000 points as the collecting goal, you might as well get a bit of everything that's Goblin-y. It's not like you don't have points for it-- Squig Hoppers are the most expensive thing goblin-related you can bring, and they're still cheaper than a lot of armies' elite troops on a model-by-model basis.