The basic answer to the original question is that a 21mm carb to 19mm intake isn't ideal but won't be quite the catastrophe that rebelmoby made it sound like, at least in my opinion.
Going 26mph doesn't immediately mean that your ped has a problem, you might have a 1.5hp engine. Check the hp rating before you spend any time looking for a problem that might not be there. The larger issue is trying to put all that on a stock cylinder at all. This is a bit of a simplification but the carb, cylinder and pipe are meant to work in balance with eachother. You can't radically increase the sizes one at a time.
On a side note, I don't know what conversation you had with the seller but no carb is "bolt and go" for any ped. A new carb will always require tuning at the very least. PHBGs are more complicated to tune than pretty much any other carb, read the wiki article specifically written about tuning them.