Department of Energy types are braced for a mini-invasion from the Pentagon.

DOE's new chief, Charles W. Duncan Jr., is quietly assembling a team of former colleagues from Defense (where he held the No. 2 job) to come into troubled DOE and set up a hard-nosed management group with chain-of-command accountability.

To help ease out senior bureaucrats, DOE has received permission to extend, through September, its early retirement program. It allows workers to volunteer for retirement at age 50 with 20 years of service, or at any age with 25 years service. Insiders expect a number of officials who didn't exercise the early-out-option this summer will have a change of heart over the next 30 days.

Among those expected to join Duncan, a one-time roustabout and engineer for Humble Oil, at the Energy Department are:

Bernhardt K. Wruble, currently director of the office of government ethics at the Office of Personnel Management. Wruble has been at the OPM only a short time. He came from Army, where he was principal deputy general counsel.

Deanne C. Siemer, now general counsel of the Defense Department. She is expected to become a top aide to Duncan, and trouble-shooter in DOE.

Brig. General Colin L. Powell, military assistant to Duncan at the Pentagon. Powell, Wruble and others have been detailed temporarily to DOE as part of Duncan's transition.

Equal (Pay) Treatment: Sen. John H. Chafee (R-R.I.) made a strong pitch last week for a 7 percent pay raise for federal and military personnel. He said government personnel ought to be entitled to the same voluntary wage guidelines as other American workers.

In a message to President Carter, Chafee said Congress can hold itself and top political appointees to a 5.5 percent boost, or nothing this year, if it chooses. "But the average federal worker or soldier should not suffer any more than anyone else. They should receive the 7 percent increase."

Bigger October Pay Raise? There may be a ray of hope for something more than 5.5 percent this October for federal and military personnel. President Carter set the 5.5 percent ceiling in January. But that was based on his 7 percent wage guidelines for industry.

Those guidelines have been broken repeatedly. And the cost of living has gone up much faster than anybody anticipated. With that in mind, the Council on Wage Price Stability is working up new wage guideposts which could allow the President to authorize a bigger raise for government employes.

All this is based on the assumption that the new guideposts take into account that government-military personnel were held to a 5.5 percent raise last year, even though the government said they were due more than 8 percent to match inflation and industry raises.

White House aides said a package of option papers dealing with federal-military pay was flown to Camp David yesterday. They declined to say what was in them. They still expect the President will make a major pay statement this Friday. The nature of it is still unknown.

According to government pay data, federal workers are due an average of around 10.4 percent this October to catch up with industry, and make up for being shortchanged last year. If President Carter decides to grant less than that amount, he must submit an "alternate plan" to Congress by midnight Friday. Congress then has 30 days to overturn his alternate plan (approving the higher amount) or it goes into effect automatically in October.

Whatever the amount, it is supposed to go into effect, Oct. 1 for military personnel, and begin with the first pay period on or after Oct. 1 for the million-plus white collar federal workers.