Rim width (internal) vs tyre size?

well I run open pros (15mm), Open 4s (13mmish)and have used a 32mm easily, on the Open 4s it properly looks like a lightbulb but even cornering around a mini roundabout at 17mph didn't cause any issues.

A 15mm internal width is fine for a 28mm, any wider and you will end up with a 28mm measuring when inflated a fair bit wider, a 17/18mm internal width rim will likely make a 28mm more like a 30mm and a 25mm more like a 27mm.

The standard rule of thumb the great tech guy at CTC, Chris Juden, used to give was 1.4 up to 2.2 times the internal width, with 1.8 times internal width optimum. So 17mm internal will take anything from 24mm to 37mm tyres with 30mm optimum. I was running 25mm Conti GP4ks on 17mm rims (internal) until recently & they were great.  In the winter I'd switch to CX tyres (33mm, UCI rules) & they worked fine too. I've recently switched to 28mm Schwalbe Pro 1 tubeless for the road & they're even better than the Contis. So, yes, 17mm or 18mm will be fine with 25mm tyres but have you considered going a bit wider on the tyres?

The trusty fallback is the ETRTO rim width chart. I have wheels with 19 mm internal width, and as per the chart, I shouldn't be running anything narrower than a 28 mm road tire. So far it's worked out for me, as 28 mm rubber has slowly become more available.

Mind you, the rims' internal width seems to have blown my effective tread width to almost 30 mm across from the nominal 28, so there's that as well.