First of all, quit beating yourself up. Mistakes happen and you didn't kill anyone. PEG tubes have to be replaced periodically as they break down from use, meds, and stomach acid. Always check with your pharmacist about whether a medication can be crushed to be given per tube. Was the patient able to swallow the pills? If not then it was not appropriate for the physician to prescribe a med that could not be crushed and given per tube.
In order to keep something like this from happening in the future, remember that water is your friend when it comes to feeding tubes, whether they are PEGs, Js or nasal feeding tubes. Always flush with at least 30 ml of water before you give meds. Give each med separately, crushed and mixed in a little water, usually about 30 ml is enough. Flush with water between medications and before you resume tube feedings. If you are doing bolus feedings, check whether the medications need to be given before or between boluses, and always flush before and after each bolus. Most patients need anywhere from 500 to 1500 ml of water outside of their tube feeding product for hydration (if they cannot drink water or are not on IV fluids), so you should have plenty of water to use for medication administration. Just subtract the amount you've used for meds and give the rest as a flush.
There are commercial products that work well to unclog feeding tubes. The Clog Zapper is the one our team uses and we've had quite a bit of success with it, especially with J tubes and PEG tubes.
You can also crush a bicarb pill and a viokase pill, mix them together in warm water, and use that to try to unclog the tube. It's much cheaper than the Clog Zapper and utilizes inexpensive medications that are likely already in your hospital pharmacy (sodium bicarbonate and pancreatic enzymes). I don't think it works quite as well though.
Some people will tell you to use Coke. That is not a good idea, as the carbonic acid and sugar is hard on the tube and can cause it to fracture, especially over time if the tube becomes clogged often and Coke is used to unclog it.
BTW, welcome to nursing!!