The Montgomery County Council agreed yesterday to cut seven cents from the real estate tax rate. The move will still mean higher taxes for two-thirds of the county's homeowners, but it will provide a $5.1 million budgetr surplus described as a cushion against the possibility of federal aid cuts.

On another matter, the council appointed David L. Scull to serve the remaining 18-month council term of his mother, Elizabeth L. Scull, who died of cancer May 29. As expected, he had the support of five of the six council members, who cited his record of service as a member of the state legislature and his familiarity with the county where he was raised.

The council's seven-cent reduction in the general tax rate will bring it to $2.27 per $100 of assessed valuation. But the tax bills mailed to Montgomery homeowners include several additional levies for such things are fire protection, recreation and special taxes imposed by localities. Those levies will bring the overall tax rate next year to an average of $3.27, although some homeowners will be charged higher or lower rates, depending on where they live.

The tax rate, scheduled for final approval Tuesday, also will not affect all homeowners equally because of the system under which a third of the homes in the county are reassessed each year. For example, the owner of a $100,000 home that was assessed at that figure in 1979 and reassessed last year will get a tax bill that is $128 higher this year; the owner of a similar home also assessed in 1979 and reassessed this year will have a bill $204 higher. The owners of the third of the homes in the county that will not be reassessed until next year can expect slightly smaller tax bills. In the case of a $100,000 home, the bill will shrink by $17.

County Executive Charles Gilchrist rcommended cutting only four cents from the general tax rate on March 1, but revenue estimates were higher than expected, allowing the council to make a deeper cut while budgeting the greatest surplus in recent years.

Council members supported the surplus as a cushion against federal funding cuts, citing as an example an expected $2 million cut from the country's $6 million in CETA (Comprehensive Education and Training Act) funds.

"This is fairly thin ice in a troubled climate," said Council Member Neil Potter, defending the surplus. Gilchrist said he supported the council action.

In winning appointment to his mother's council seat, Scull got the support of all council members except Rose Crenca, who cast her vote for Thomas E. Bratton Jr., treasurer of the county's Democratic Central Comittee. Crenca argued that Scull doesn't meet a county charter provision, which requires residence in the Silver Spring/Takoma Park district his mother represented.

On Thursday night Scull moved from his Chevy Chase home to his mother's house in Silver Spring and said he has changed his voting registration and driver's license address. He said his wife and 5-year-old daughter will join him as soon as the house is renovated.[WORDS ILLEGIBLE] mother's district immediately before his appointment was "manipulative and devious. My feeling is that the 5th District is not being given a fair shake."

Potter reminded Crenco that all seven council members are elected at large, although five are required to live in different geographical districts. He said her comments displayed "provincialism" and that all council members are sensitive to the needs of the 5th District.

When Scull is sworn in on Monday he will resign from the Maryland House of Delegates. The County Democratic Central Committee will select his replacement in the legislature within 30 days.