Might I suggest making your own combo concentrate and syrup? Its way fun (I'm a noob myself) and super easy. Take 4 cups of water and add various roots (sassafras, wintergreen, anise, etc) and cook for a few minutes and strain. Then return to the stove and add equal amounts suger and wala, syrup...
I used a teaspoon (maybe a half, I think) of guar gum in my root beer syrup concentrate I make to get that good smooth mouth feel. I added it at the end of my cooking so it would dissolve well. Fair warning to start with a little or your syrup will get too thick.
I followed this recipe but added hops, wintergreen root, dandelion, some vanilla, and used anise extract. It didn't even fill up a plastic koolaid container in my fridge. When I use the sodastream I just tilt and carefully pour my syrup into the bottle a little at a time until im satisfied it...