I used to use Vivillon in my Ho-Oh deck but found that it took too long for it to be worth it for energy acceleration (which just so happens to be one of the biggest hurdles to a functional Ho-Oh deck. . My current setup (although expanded) is as follows:
Pokemon (15)
2 Ho-Oh EX BKP
2 Larvesta...
the trainers all look relatively good, but you could switch out the pokemon so that you would have a few more attacking options. My current Ho-oh deck format looks like this:
3 ho-oh ex (BKP)
2 smeargle
2 dragonite ex (FF)- this would probably be the most expensive piece for you to add it runs...