What do you mean by "mixing rules"? Are you talking about requirements set by federal governments? If so, which ones? For a mixer to be able to service customers based in the U.S., it requires a money transmitter license, which means the mixer must not only register with the government but KYC for all customers (which defeats the purpose of the mixer). Other countries may have similar laws, some more lax laws, and some may not have any laws addressing the issue at all.
There is no financial institution that doesn't abide with set of law guiding the activities of the institution and before tumbler or a mixer will operate in the cryptocurrency ecosystem, it must be issued with a license and the license would be giving to it when they have accepted to obey the rules and meet up the requirements to operate. This is one of the law a mixer must follow
Mixing large amounts of money may be illegal, therefore a good mixing company will operate in the financial law of not mixing large amount of funds because criminals will use it to launder money.