pestilus is a better choice than trumborr, since he adds hp burn, a crit damage buff, and some additional healing on top of leech. scabrius could also work, since he gives a second source of poison (though you'd want to make sure he's fast enough to keep the leech debuff up constantly). leech only works with direct damage, so poison (as well as hp burn and bomb debuffs) does not trigger any healing.

is warmaiden equipped yet? she may work better than bellower in your current setup.

Right, Pestilus might actually work for me better. No, Warmaiden is like non-existent anywhere in my teams. I just leveled her up to have a DEF down champ, but I don't have enough gear to gear up all of my champs yet.

Also, should I care about speed tuning yet or not? Because my team is nowhere near to be tuned, and Skullcrusher's ally protect falls off way too regularly. 

at the moment, i wouldn't bother. you're still at the stage where your champions get used in all areas of the game, so building them specifically for a single area doesn't make sense. on top of that, skullcrusher is the only champion that's likely to be in your cb team long-term and there's not much point in speed tuning a team thats all going to be replaced in the next few months.

you could try tuning skullcrusher to go right before the boss if you wanted to, but having apothecary in the team will make that tricky and you'd have to readjust his speed every time you moved up to a higher difficulty.

All right. Thank you for the tip! I'll start building Pestilus to 50, but I don't think I should max him

Don't make to many lvl 50 champs. That slows down your progress in the game. Concentrate on the champs that are supposed to be lvl 60. If you don't see a champ at lvl 60 in the near future, don't build him at all.

Farm masteries for your champs and bring Aothar to lvl 60. Bringing a lvl 50 champ without tier 6 mastery instead of a lvl 60 champ doesn't help your team.