The security officer process is guided by contract security or other firm oversight. Hence licensing rules are identical for the security guard or officer in the armed or unarmed context.
As for weapons and permits, the State of Maine does not regulate handguns, rifles and shotguns.
Even concealed weapons are freely permissible.

See: Maine State Police permits

In Maine, individual security officer licenses are not issued unless tied to a contract security firm.

The security guard business, whether individual, partnership, corporation or other business form is the primary party responsible for the licensure of individual guards and officers.

The business is required to conduct background reviews and assure that the officer meets the defined criteria of a security officer.

Full coverage of these requirements is outlined in the Maine Security Guards Handbook.

32 §9404. LICENSE REQUIREMENT; EXCEPTIONS

1   No person may act as a security guard without first obtaining from the commissioner a license to be a contract security company.

2.   This section does not apply to the following:

A.   Any proprietary security organization or any employee thereof;
B.   Any person employed by a person possessing a valid license to be a contract security company;
C.   Any person possessing a valid contract security company license granted under any prior existing provision of law of this State, provided that upon expiration of that license the person shall be governed by this section.

The fee for a private security license application is $400, of which $100 must be submitted with the application and $300 must be submitted upon issuance of the license.

See: Title 32 §9404. License Requirement; Exception

You must be licensed in state- there is no reciprocity.

32 §9410-A:
Security officer must be at least 18 years of age

32 §9410-A:
Security officer must be a citizen or resident alien of the United States

None required

None

A concealed handgun permit may be required.
See: Laws Relating to Permits to Carry Concealed Handguns (PDF)

Maine does not permit the carry of dangerous weapons by security personnel at labor disputes.
See: 32 §9412. UNLAWFUL ACTS at (5)

Maine does explicitly mention the right of security officers and guards to carry weapons in the transport of money. The statute states in part: 32 §9416.
See: 32 §9416. Firearms

A person who acts as a security guard may, while he is in the performance of his duties in that capacity, carry a loaded weapon in a motor vehicle which is being used to provide armored car service or which is otherwise transporting things of value, provided that any weapon carried is not concealed.

The State of Maine extensively reviews the moral character of contract security applicants but not individual licenses.
The responsibility to review character is an employer based responsibility. The Maine Security Handbook 32 §9405 1-A (F.6) states:

A list of employees as of the date the applicant signs the application who will perform security guard functions within the State.
This list shall identify each employee by his full name, full current address and addresses for the prior 5 years and his date and place of birth, height, weight and color of eyes. For each employee on this list who will perform security guard functions at the site of a labor dispute or strike, the applicant shall have previously investigated the background of the employee to ensure that the employee meets all of the requirements to be a security guard as contained in section 9410-A, subsection 1. If the employee meets all of the requirements to be a security guard, the applicant shall also submit a statement, signed by the applicant, stating that the applicant has conducted this background investigation

None required

32 §9410-A:
The applicant cannot be: a drug abuser, drug addict or drug-dependent person

32 §9410-A:
The applicant cannot be potentially dangerous to himself or others as the result of a mental disorder

According to 32 §9410-A:

The applicant cannot have: been convicted of a crime punishable by one year or more of imprisonment or, within the past 5 years, any crime enumerated in section 9412; 3 or more convictions for crimes punishable by less than one year of imprisonment within the past 5 years; Is not a fugitive from justice; has not been adjudicated to have committed a juvenile offense involving conduct which, if committed by an adult, is punishable by one year or more of imprisonment or, within the past 5 years, a juvenile offense involving conduct which, if committed by an adult, is a crime enumerated in section 9412.

Furthermore, the applicant cannot have been dishonorably discharged from military service within the last 5 years.

See: 32 §9410-A. SECURITY GUARD QUALIFICATIONS

The Security Officer employer/licensee is required to conduct an extensive background check of all of it employees with 60 days of employment.
The rules read in part:
Within 60 days of the date that the employee begins to perform security guard functions within the State, the licensee shall complete this background
investigation and submit to the commissioner a statement, signed by the licensee, that the licensee has conducted the background investigation and that the
employee meets the requirements of subsection 1. This statement must be submitted to the commissioner before an employee may wear, carry or use a firearm in the
performance of security guard functions and before an employee may perform security guard functions at the site of a labor dispute or strike.

See: 32 §9410-A. SECURITY GUARD QUALIFICATIONS at paragraphs 3-4.

None required