When Rek'Sai Breacher comes in, do other units get Accelerate and not have to pay 1 to use it?

No, Accelerate is not free.

According to Rule 805.1.a, Accelerate is functionally short for: "As you play me, you may pay [1] Energy and 1 Power as an additional cost. If you do, I enter ready."

When you control Rek'Sai, Breacher, she grants the Accelerate keyword to friendly units played from anywhere other than your hand (such as tokens created by abilities). This gives you the option to pay that cost to have the unit enter the battlefield ready.

Here is how the interaction works:

• Optional Cost: You are not required to pay the cost. If you choose not to pay it, the unit will enter the battlefield exhausted (the default state for units).
• Payment Timing: If you want the unit to enter the battlefield ready, you must pay the [1] Energy and 1 Power during the "Pay Costs" step of playing that unit (Rule 805.2).
• Cost Details: Because tokens (and most units affected by this) have no domain or multiple domains, the Power portion of the cost can be paid with [A] (a Power of any domain).

In summary, Rek'Sai, Breacher grants your units the opportunity to use the Accelerate mechanic, but you must still pay the associated cost if you want them to enter the battlefield ready.