Aliases — if you're referring to pen names, or perhaps actor names, or other names assumed for certain activities — have their own numerology energy, just like any name does.

Using the alias would bring additional energy to a person, augmenting the energy provided by the person's birth name — similar to how a new name augments the birth name when a name is changed. The birth name is augmented, not replaced. There may be some incompatibility, but I don't see where the chaos would come in.

Understand that the energies represented by numerology numbers are tendencies. There is no leash or pen; and there is no predetermination. Free will always rules and it can go with the tendencies, somewhat with them, or a different direction altogether.

Yes, there is difference between the number 81 and the number 18. Those links lead to articles talking about each of the numbers.

They both reduce to the number nine, which represents the basic energy of the two numbers. But the digits within those multi-digit numbers also provide energy. The first digits provide more energy then the subsequent digits. The articles at the above links have graphs to show the differences.