A: As old-timers know, years ago a lot of fly rods on the market that didn't cast well were on the market. But today it is rare to find one that is not a decent casting tool. Almost any fly rod you buy today will cast at least the line size recommended and one lighter or heavier. A good fly-caster can probably use most modern rods to cast lines two sizes lighter or heavier than the one recommended.
As soon as you shorten or extend fly line outside the rod tip, you have changed the casting weight. I would not worry about lines that are designed for half weight.