I have used future on nearly every kit in the last 6 years and never a problem. The cracking may be that it was used over a laquer or enamel that had not sat for many days to gas out the solvent before it was encased in future, causing orange peel. you always should let a hot paint set and cure before glosscoating, maybe a week or more. that is why it is better to apply hot paints over acrylic and not the other way around.
Yes it may darken it some, but I have not seen it to be too significant. I cannot comment on other acrylic clearcoats. i thinh the darkening properties has to do with light refraction though it, not the future darkening it.
Let us know what you choosen and the result.