There are two common cap sizes, #11 and #10. As you note, Rugers take the #11, which is generally the size used for ML rifles & shotguns. Most pistols take #10, which are smaller.
There are several choices;
1. Get some # 10's and try them. This would be my first option.
2. "Pinch" your #11's before capping, so they will stick. This is a temporary expedient to get you making some noise.
3. Uberti pistols usually take the Ruger nipples. if you have a Ruger, try it out.
Someone else may chime in, but there might be nipples available that fit Piettas, and take #11 caps. If someone knows, I would be interested as well.
If you get SHOOT! magazine, look for the article MAKING PERCUSSION REVOLVERS WORK, by Alan Lucky, aka Long Tall Texan, Vol 39, Mar/Apr 2006, page 96