Man, try to think in terms of the story. In the actual manga Vista was enough to hold Mihawk. Even if not the best it would be enough to train him for the timeskip. Him getting his growth on his own would actually be cool.
About Croc, again, try to read what I actually wrote. I will try to repeat in a different way: in a hypothetical scenario where Mihawk doesn't exist, if Oda wanted to make Buggy and Croc join together it could happen based on the fact that they had already be partners before and Croc was show to be able to be amicable with subordinates. This is a possibility if, again, the Cross Guild was something so important for the manga.
Got it? You may disagree but, if you are talking about hypothetical scenarios, try to answer based on the premisses of the scenario at least.