It states on their website that it is 'tiny boot approved' and fits into a fiat 500. Will this also fit into the boot of a mini convertible, which is slightly smaller?
I think it won't, honestly. I mean, it is a tight fit for the Fiat 500, especially with that two-piece fold (when reclined, ot when going into a small car trunk) which will mean a little bit of fiddling to fit all inside. For an even smaller boot, it will just be a too much of a fight I believe. The best is of course to measure your boot and compare with the folded dimensions, but I personally would go for a smaller stroller with a convertible's tiny boot. E.g., a Cybex Mios.
Your -very berry- Eli.