Is RC4 +XOR secure for small data?
Disclaimer: the only reason to go this way is if you have an 8 bit CPU. Otherwise, select AES.

Sending nextKey in the data packet is silly. What you want is key + iv. Look up the CipherSaber 2 variant of RC4. This covers the attack used on WEP just fine. Bear in mind though this wants a good RNG (probably hardware RNG) and 10 bytes of iv. Unless you can store the iv counter, there's no good way to reduce it. There are other possible attacks on RC4 though, the most worrying is the one that can be used against 1 billion messages to reveal the first 128 bytes of message. A plausible workaround is simply eating the first 256 bytes off the stream (don't bother sending just discard).

Your key length here won't be 64 bits though. That's too small. You need at least 128 bits.

We do have to keep in mind that RC4 and all its variants are weakening. There is simply no good explanation for the fact it is as strong as it is today. It really should not be. The next attack might doom it.

EDIT: Answer no longer good. A recently published attack against RC4 is no longer mitigated by the CS2 variant. I was not expecting this.