"...common variety seedling cacti...". What are those? If you mean desert cacti, I suspect you will find they (epis) are poor stock material. The reason for this is the wild difference in growth rates. Jungle cacti (epis and others) make lots of plant mass because they grow almost all year and have water available at least weekly almost all year. Desert cacti grow slowly, have few nutrients available (poor soil, rocky soil or a combination), and what nutrients are around are only slowly transported with a little water.

If your goal is to speed the growth of a desert cactus with an eye to regrafting it again later, it might work. This is already done, and on a commercial scale, sort of, witness the mass production of "Moon Cactus" grafted onto Hylocereus stock. The result is a short lived plant with the stock usually dieing and the scion needing to be regrafted to live.

So far as trimming the epi, go ahead and trim away. It won't hurt the plant. Making new plants is so easy that if the plant gets unwieldy or looks like homemade sin, wait until spring and multiply it. Although they will root in the winter if conditions are right, spring is easier for root growth. I'm assuming you have experience with grafting, rooting epis will be a cinch.