> I just want advice on how to keep things weird while growing.
That one was easy: get everyone that is new involved in the weirdness. Culture isn't strictly policy, it's people and it's your job to make sure that the culture doesn't die. Involve new people in the culture. If the new management gets in the way of the culture it is your responsibility to communicate that risk.
> The HR manager scares me, although I am not sure why.
Have lunch with them. That HR manager is more scared than you are. They are coming into a company with close friends and a culture that they have no idea about. Be inviting and friendly.