The tax-rate reductions for individuals in the sweeping overhaul bill the Senate will consider next week would cut federal revenue by $2.5 billion in fiscal 1987 if it became law, according to the official Senate Finance Committee report on the legislation released yesterday.

The small decline in revenue from a package that slashes the top tax rate for individuals nearly in half, to 27 percent, and sets one other bracket, of 15 percent, was apparently a result of two factors. First, the rate reductions would not be effective until the last three months of fiscal 1987, which begins Oct. 1. Second, the revenue figures seem to assume many taxpayers will sell their stocks and tangible assets in the current tax year, to take advantage of the low tax rate on capital gains before it is repealed.

Despite the slight revenue loss from the rate reduction, the entire Senate tax package would bring in $22.8 billion more in revenue than the current system in fiscal 1987, and $7.4 billion more in the preceding year. Revenue would be $20.7 billion less than current law in 1988, $21 billion less in 1989, $400 million more in 1990 and $11 billion more in 1991, the document said.

The committee report, not the specific language of the legislation, was released yesterday. No details were provided of the "transition rules," which determine how the new tax plan would take effect and outline the exceptions and exemptions from the limitations on tax breaks in the bill.

Much of the $100 billion in tax increases on corporations that the legislation would impose over five years comes from four provisions: repeal of the investment tax credit, $130.8 billion; changes in inventory deductions, $34.3 billion; accounting changes, $50 billion, and a stiff new corporate minimum tax, $33.3 billion. Because those revenue increases would be partly offset by corporate rate reductions, they add up to more than $100 billion.