I only ever put a singlet under outfits on my boys when it was cold
I would only be using singlets in this heat if it's the only thing you're putting on them! General rule of thumb is one layer more than you are wearing but I honestly think that's more for Northern hemisphere/Aussie winter. My LO is currently in a short leg/short sleeve onesie and we're inside in the aircon. Seriously though, you know your bub and they're at the age now where they can make it known if they're uncomfortable (at least my LO does - he doesn't like being too warm). So trust your gut and if you don't think they need the singlet, don't put it on!
@Beth yeah the one layer more than us is what is tripping me up! Thank you!
At this current weather a little onesie, or a singlet and a nappy, is all they need :-) my little one was getting heat rash in anything more