The House leadership indicated yesterday it probably will allow the Carter administration a separate floor vote on its plan to revamp the pending tax-cut bill - despite earlier reluctance by House Speaker Thomas O'Neil (D-Mass.).

After a flurry of conferences with key House members late yesterday, O'Neil reportedly decided tentatively that the administration's new thirteenth-hour compromise should be declared in order by the House Rules Committee today.

At the same time, O'Neil is said to be leaning heavily toward permitting a separate floor vote on the GOP-backed Kemp-Roth tax-cut plan as a possible substitute for the Democrats' pending bill. Democrats now feel they can beat it.

Still undecided was whether the leadership would back a request from liberals for a separate vote on a proposal to offset next year's scheduled Social Security tax increases. Democrats are bitterly split over the issue.

The developments came as the Rules Committee began hearings yesterday to decide which amendments should be allowed when the tax bill reaches the floor. But the panel broke up abruptly after only 1 1/2 hours.

Although the committee quit ostensibly because some of its senior members had other obligations, the move gave House leaders more time to thrash out their differences. Sources said the panel still hopes to clear the bill today.

The new plans by the leadership marked a turnabout in O'Neil's earlier position. On Monday, the speaker reportedly was giving serious consideration to denying the administration a chance to offer its floor amendment.

At the same time, moderate and conservative Democrats held another meeting yesterday to voice their support of the pending bill which was reported out formally by the House Ways and Means Committee.

Although the session was closed, sources said the Democrats got into a heated debate with Treasury Secretary W. Michael Blumenthal, who was there to seek support for the administration's proposals.

Several participants said the Democrats chided Blumenthal for not having proposed his compromise earlier.

Yesterday's hearing at the Rules Committee was limited to a routine description of the pending bill by Ways and Means chairman Al Ullman (D-Ore.). Ullman supported his panel's position to oppose a vote for the Carter plan.

Rep. James Burke (D-Mass.), sponsor of one of the Social Security measures, also appeared to urge approval of an amendment providing payroll tax relief. Burke left a hospital bed to deliver his testimony.

The Ways and Means measure would provide $16.3 billion in tax cuts beginning next January - including sizable reductions in capital gains taxes and rate cuts for middle-income and upper-middle-income groups.

The administration's proposal would reshuffle the tax cuts for individuals to provide more relief at the lower end of the scale and less for high-income taxpayers, and trim some capital gain cuts for those with tax shelters.