Mixing formulas

Is it okay to mix a different brand of formula into the baby's regular formula? I ask because my son has been regularly using Enfamil Gentlease for 3 months now, but we have some sample cans of formula that are unopened and were given to us by friends who did not need them. I would like to use them since they are free, but I don't want to abruptly change his formula to use a sample can for just a couple days and then change back to his normal brand. So could I blend in a sample can into his regular large tub of Enfamil, that way it's about 75% Enfamil and 25% other...