Just my opinion(s), but:
1. "second hand knowledge is wrong 90%" this really depends on where you're getting your information. This is a pretty blanketed statement...I'd take second hand knowledge from notable scientific experts anytime.
2. Male umbees will generally be larger than their female counterparts. This doesn't mean females will not be larger in some cases. This is true with many species of cichlids. I've bred alot of different cichlids, and within some groups, females grew larger and more dominant. Again, usually, the males are larger.
3. SiestaSkyy, you say before you can sell to the pet shops, "brokers" buy the bigger ones first. Can you tell me who these brokers are? Are you referring to large fish farms and/or wholesalers? If this is the case, then they in turn would sell to stores or other wholesalers/importers. Regardless of who gets these large quantities of the larger sized fish, they have to end up somewhere. So where are they? (I'm not trying to dispute your claims, I'm honestly just curious as to where these fish end up? If not local (US), are they being shipped internationally?)
The rise in umbee interest at this time has far surpassed the surge of interest a couple of years ago when they were popping up. I recall sending a bunch of 2 inchers and fry to the Michigan area at that time when I thought the umbee craze peaked.
I have kept, bred and raised umbees for years, and I never understand why there is so much controversy with them, particularly in the recent years. As far as the sexing of the umbee, if you are serious about keeping this fish, patience is key. Raise a few fish for a good year or two and you will find out if you have a male or female. If you don't want to "waste" your time raising an umbee, only to get a female, then you should re-think purchasing these beautiful fish in the first place.
My 2 cents.