If you breed silings You take the good and the bad. If both parents have good gentics you'll be fine. But if both carry recessive/hidden bad genes like messed up spines, you'll see alot of that in the fry. It depends on the gentics.
if you got a pair of fivestars they should be in good condition and will breed for good fry.it doesn't cause anything bad in and of itself.
I personally love the look of fighters, they looks different then the ones that have been crossed to HM's for many generations, and their colors look more natural. If you do spawn the pair I would love to see a spawn log from you about it, with notes on how the agression they're bred for specificly effects breeding(for better or worse).