Kumho 857s, pressure?

I don't think there is any reason why one should not run trailer tires at anything less than max pressure per the sidewall. Trailer tire problems are very often caused by under-inflated tires. My guess is that your rims can handle the pressure if they are the typical steel rims. My rims don't have a max pressure designation on them (that I can find) but I run at 65 psi so my tires last. I have not heard of too many problems from tires (or rims) run at max pressure.