Chip:
To answer your question in reverse order:
Sure, 1% hydrocortisone is safe, though it probably won't help much. Most likely, the case you treated was mild and going away anyway. But that's OK.
Poison ivy can't scar, because it just affects the epidermis of the skin.
Once you're allergic (and most people are), you'll pretty muchy always break out. If you have the kind of violent reactions you describe, you really need to avoid vegetation (wearing clothes, etc.) Eradicating the plant is essentially impossible, and it's always mixed in with other vegetation, so you never see it.
So keep your shirt on. Literally.
Best.
Dr. R
when you get an allergic reaction to anything, in most cases the reaction becomes worse with each exposure, If I were you I would be very cautious of your environment and carry an epi pen.