Why would it have been disastrous?
Why did Tibbs advise against the Dalmatians staying to fight? I mean, just a scene earlier, Pongo and Perdita have NO problem handling Horace and Jasper on their own, and now they have a bigger dog (Colonel), plus a HORSE (Captain). And when they leave, Colonel starts barking and growling at them anyway, and Captain practically kicks them through the barn's WALLS.
So...how exactly would this have backfired on them fighting them TOGETHER if they all could handle Horace and Jasper separately?