1500 points is a big army, and pretty much any ally will do, it just depends what you want the army to look like. Looks like you have a good start with Rohan, so you might as well start there, maybe add Erkenbrand as a good mid-range hero (cheaper than Eomer, and as effective...in some ways moreso). All RoR can now use their bows, which is pretty nice.
Army building takes a while, you generally won't get a lot of responses unless you at least do the basic work yourself and then ask for opinions on your list. You can play around with army lists with this handy spreadsheet:
http://www.thelastalliance.com/viewtopi ... &start=130
(check the signature)
And usually you'll want to tailor your army to what you personally like. There are a lot of ways to make a "powerful" army, the great thing about the game is how many ways there are to do that. However, there are a few general tips:
First, learn the game with what you have. That will help guide you towards the army you want.
It doesn't look like many of your models have spears, a box of wood elves can help with that. Spears are important, if you don't have them you need banners or some other way to add dice to a situation. Or if you're focusing on high elves, you might want some high elf with spear + shield, but they're metal/finecast, and pricey.
Even though mobile shooting has taken a hit, you will still want to maximize your bows.
In many ways the defence sweet-spot is D3 and D5 (and D7 for heroes) for good armies, because evil shooting is S2 and S4, and many armies have S4 warriors. So Galadhrim without shields are often wasted (dying twice as easily) unless you're up against evil men, or basic goblins and orcs. This can mean high elves with shields often don't add much, but there's something comforting about the look of them so I don't convert
The heroes you have are all good for their points. Personally I skip Haldir...if I want a bowman I take Legolas, and if I want a fighter I take Rumil. The Twins are great, more effective than Glorfindel for warrior-chewing, but Glorfindel is great for handling wraiths and other big nasties.