You won't get halfsun first time . . .
It takes generations of line breeding to get a new tail type with any refinement and consitency.
In your first spawn you'll probably get a bunch of combtails, HMs, CTs, "misc" (stuff that is too short, a weird shape, has odd branching, etc).
eventually, if you persist down the line and breed the best fry together you might get some nice halfsuns.
Have you got any proof? I have just read a handful of sites saying nowt about *generations of line breeding*
If it was that easy to get halfsuns, you'd see a heck of a lot more of them around.
When breeding any new line, you will get culls. The greater the difference between the sire and dam and the more 'extreme' the fry you are hoping for (so, CT + HM = HS), the more fry you will get that really, really don't make the grade. It's basic GCSE genetics.
You know, if you take a warmblood and a arab and want a racehorse out of them you won't get it first time. Same with cats, dogs, birds, bettas . . . if you want to achieve any specific goal by putting two rather different animals together, you'll need to persist down the line and in the meantime, you will get a lot of fry that take the bad aspects from the parents. You can't expect all the fry, even the majority, to have the traits you want them to have.