I have a 20 year old-ish metal 'Square D' consumer unit in my condo, just wondering how 'universal' is the breaker, do I have to use a specific same brand breaker that fits inside the box, or does any brand breaker mount universally like on a DIN rail, or will there be issue with face plate size? 

I'm thinking about adding separate RCBO for water heater and some Wifi-controlled breaker for the aircon. 

Your CU is almost certainly of the "plug-in" type which means you're limited to Schneider/Square-D, BTIcino, Chang and a couple of others.

 

You can certainly get RCBOs to fit, anthing else you're looking at an external box.

 

"Wifi-controlled breaker"  I Googled that and yup, they exist.  But I don't see anything describe what the wifi function is/does.  (?)  S'plain it to me Lucy.

51 minutes ago, bankruatsteve said:

"Wifi-controlled breaker"  I Googled that and yup, they exist.  But I don't see anything describe what the wifi function is/does.  (?)  S'plain it to me Lucy.

You can trip and reset a wifi circuit break from a smart phone. The breaker contains a small motor to drive the reset.

Great for resetting breakers while absent and not knowing if the house will burn down.