Infantry heavy lists are easy enough to come by with the Skorne, but since you are specifically using the Arcuarii and Karax I'll do what I can to help. The Karax are amazing, easily better than most shieldwalls because ranged units can shoot past them and AoE's cannot deviate past them (plus they don't take blast damage) not to mention most troopers needing to charge them to damage them in shieldwall, but then they have the set defense bonus. The Arcuarii on the other hand are hard to endorse. The harpoons are really only useful to take the occasional pot-shot if they cannot make it into melee. With their low
RAT it's unlikely they'll hit a lot of troopers, but being able to hog-tie a heavy beast/jack can be useful given certain circumstances. If you have the option of charging into combat, do it because they are more accurate and will do a lot more damage in melee than with their harpoons.
As for some beasts, a drake and krea (or two) are a powerful combo. The krea's animus is one of the best in the skorne arsenal if not the game giving -3
def and 1/2 speed. The drake get's a stud bonus being with a krea and it's gaze attack is amazing as well, especially mixed with it's animus. Cast it on some venators and have them shoot through the Karax to push them back and either allowing a charge attack, or just pushing them out of combat to move them about as needed without worry of free strikes. Either that or have a Titan Cannoneer blast away in front of the Karax preventing the AoE from deviating into your army. Those are really the only particular synergies I think are worth noting, and anything else you want to fill out your list works just fine. However, an auto-include for me is the Shaman because of it's Spirit Blade animus being pretty much the only anti-spellcasting ability of the Skorne army.
Skorne has a fair share of locks that boost infantry including Makeda, Zaal, Mordikaar and possibly Hexeris if even just for Death March. So really, it just depends on what other models you want to use. Since Xerxis is a Cataphract himself, I like him with either Cataphract if for no other reason then theme. Makeda can let the Karax run in shield wall or boost their defenses and Carnage boosts attacks of any models you want. Zaal works well with cheaper units to maximize his feat and to power ancestral guardians, but is still effective with Cataphract. Mordikaar is nice because of Spirit in Amber allowing him to temporarily revive the defeated Cataphract or Karax (and with their
CMA the more the better).
I already went into some detail about the Cyclops combo's here:
http://www.dakkadakka.com/dakkaforum/posts/list/250970.page
If you have any doubts about Molik Karn, I would recommend him at every turn. He has great
MAT for a heavy and can trample with the best of them especially with Guided Strike. His animus gives him a speed boost (out of turn), and with Fate Ward (mixed with the agonizer) makes him a nightmare to take down.