New Bumblebee Cichlid - Need Advice

Hello Everyone!
I recently purchased a 2nd hand 55 Gallon tank from a rank amateur, and along with it came the lone survivor of his dying fish population - a 2" long Bumblebee Cichlid (crabro). He/she has been in my 10 gallon quarantine tank for the last couple weeks and has nearly doubled in size. Although I've been a fish keeper for nearly 40 years, this is my first foray into African Cichlids, and I need help figuring out what to do with him/her.

First of all, how soon can I sex him/her and can that even be done with a single fish? He/she is about 3" long now, growing fast, is very yellow all the time and has no spots on the anal fin. From what I have been able to gather off the 'net, it's a girl, but am I jumping the gun?

Secondly, she can't stay in the quarantine tank forever and if I buy another large tank, my wife is gonna kill me. The 55 gallon is for two breeding pair of Angels (scalare), so that's out. I have other tanks with room enough but do you think she would get along with female bettas? How about non-breeding, full-grown scalares? Are female Bumblebees less aggressive than males? Or should I just find her a new home entirely?

I'm a little concerned the Bumblebee's respiration rate is a bit higher than what I'm used to. Could this be normal? Otherwise she seems to be thriving, as are all my other fish. My water quality is a very soft 7.5 ph which I already artificially harden with calcium carbonate and magnesium sulphate to nourish the heavy live plant loads in all my tanks.

Thanks in advance for any help,
Jim