They can be slightly nippy of longer fins, but they are more pretty in my opinion that Emperors. The "rainbow" shine is very unique and the different eye colors for male/female are kinda cool as well. The ones The Wet Spot has right now look amazing - I wouldn't wait on it if you're thinking about them.

If you have fish with long fins or are slow moving, maybe think about something else instead. These do get 2" or so at absolute full length, so they aren't a nano fish and can do some damage if they release their inner piranha!

 

I haven't kept them, but I have emporer tetras in my 75 gallon with Rainbows. They are very aggressive. My original group of 35 worked down to about 12.. and a couple weeks ago another fight broke out and one of the males lost an eye. I say that because from what I have read lacortei are more aggressive than my regular Emporers (palmeri). If I had it to do over I would've stocked 40 at one time. Adding them in smaller groups leads to different sizes and fatalities at some point.