On a forum like this anytime someone sez that you must do this or that you can't do dat , you probably can do dat . There are very few absolutes !
Many ST tires are rated for more than 65 MPH . Should you drive faster , probably not , but ya can if ya want !
Some manufacturers install tires other than ST tires . I delivered for many years cargo trailers that come from the factory with non ST tires .
Should you put "P" tires on your trailer , probably not as there are better alternatives , such as better brands or more ply ratings .
What ya do not want to do is get the " Tire Police " on your back as they will track ya down with dogs and let the air out of you China Bombs !