cuckoo syno

The problem with a lot of Synodontis Catfish now is that they are being hybridised, intentionally or otherwise, and many imported species aren't any longer what they're supposed to be!

I have Synodontis Petricola which, I hope and believe, are true ones...I bought a further two last year and they most certainly aren't !!!

So, you might even find that your 'mystery' fish turns out to be one of these dreaded hybrids!

In the main Synos prefer harder more alkaline water than some of your existing fish but if they're showing no signs of discomfort I shouldn't unduly worry.

Synos are usually kept with Malawis and Tanganyikan Cichlids - indeed that's the reason for the 'Cuckoo' common name for one of yours, some will sit on the sidelines when the Cichlids spawn and before the female Cichlid gets the chance to pick up the eggs they will dart in, eat those eggs and replace them with eggs of their own! These are then carried dilgently by the female Cichlid in the mistaken belief that they are her own eggs! If I remember rightly a video was posted a couple of weeks back showing this happening in real life.
To your other question, yes, they will become 'bolder' if you have a shoal of them but not so bold as to be 'out and about' at all times, at least mine aren't. I also have four Featherfin Synos too (Euruptus) and their behaviour is somewhat similar...although, having said that, they are first in the queue when food enters their tank! Then they become very boisterous, less assertive fish get physically shoved to one side!
Great fish, though - for all that.

Good luck,

John