That limitation should not apply anymore with 0.8.0-rc.4 and later - it should be now 622byte, but even before it was 63 characters.
So even with the legacy limits you could send a credentials string with a SSID+PWD taking up to 59 characters followed by a two digit number representing encryption and WLAN cipher
You currently can't select which of the stored credentials will be used. The creds stored last will be the ones tried to connect to first.