If I have red reksai in play and then I play rumble from champion zone. Do I have to pay an accelarate cost for that rumble to enter ready via red reksai ability?

Yes, you must pay the Accelerate cost if you want Rumble to enter the battlefield ready.

According to the rules for Rek'Sai, Breacher, she grants the [Accelerate] keyword to friendly units played from anywhere other than your hand. Because you are playing Rumble from your Champion Zone (which is not your hand), he gains the [Accelerate] keyword.

As established by Rule 805.1.a and RiftJudge FAQ #9725, [Accelerate] is an optional additional cost:

1. The Choice: Because the unit has [Accelerate], you have the choice to pay the cost to have it enter ready. If you choose not to pay the cost, the unit enters the battlefield exhausted (the default state).
2. The Cost: Per Rule 805.1.a, the cost is [1] Energy and 1 Power.
3. Payment: You must pay this cost during the "Pay Costs" step of playing the card. You cannot pay it later once the unit is already on the board.

In summary, even though Rek'Sai, Breacher provides the [Accelerate] keyword, the mechanic itself is never free. You must pay [1] Energy and 1 Power if you wish for Rumble to enter ready.