For more candy-like flavors, I'd lean more towards TFA and Capella.
IMO, Capella does bakery best, and some of their fruits are good. Capella's Vanilla Custard is the industry standard for base custard flavor. As percentage goes, I use anywhere from 4-10%, depending on the flavor. I'd say Capella is the least concentrated.
TFA does candy fruits, although they're really hit or miss in terms of what the flavor actually tastes like. For example, they have raspberry and raspberry sweet. The raspberry is awful, and strong. The raspberry sweet tastes like those chewy raspberry candies with the candy bumps on them. Their honeydew melon is perfect melon. Their cantaloupe is weird, weak, and not very cantaloupe-y. Also, TFA's potency is all over the map. Some need anywhere from 4-10%, some need .5-1%.
FA does realistic tasting everything. Most flavors are spot-on, as far as what you'd expect to taste. Most are very potent. They range anywhere from .5% to 3%.
INW (Inawera) also has an incredibly large range of flavors. There are a lot of weird mixes and nondescript oddities. I've found they're pretty hit or miss as well, but there are some gems. They're pretty concentrated as well. 1-4% would be my estimated range.
None of these percentages are a hard and fast rule, since I've used all sorts of percents, but I wanted to give a basic idea of what you're dealing with.
As far as sweeteners, I haven't used any in well over a year. They don't really add much but a sicky-sweetness, and can mute your flavors.
Remember, when dealing with flavors, less is usually more. Meaning, start at a lower percent and work your way up. Too much flavoring can kill a mix, and you end up with a weird tasting, muted failure.
