I know of more than one person who rides their upper level horses in a regular snaffle and a curb and they actually find their horses go better in the regular snaffle/curb combo. I likely will be doing that as well when I start my horse in a double but that being said, these are large warmbloods. I personally have nothing against riding a horse in a double with a regular snaffle, your horse just needs to have enough space in his/her mouth.
When I was riding my pony (14.2 hands), whose head is built much like an arabian, upper levels he went in a low port, short shank curb and a regular bridoon quite well. Like Uncleathur mentioned though about the double being too much of a mouthful: even the smallest ported bit with the shortest shanks were nearly too much of a mouthful for him.
You might need to try a few different curbs before you find the right one. Each one fits differently and each horse has different preferences. I'd suggest looking for as low of a port as you can and short shanks simply because of the limited space smaller horses have in their mouths. If your horse goes well in a french link snaffle there's no need to go with high ported bit with long shanks since it sounds like you won't need the leverage.
Last edited by Tigo; 03-06-2012 at 06:52 PM.