Can i use my own stocks (or watchlist) as a Universe in the screener?
Yes. Create a list. Then create a universe with the inlist rule.
Or create a universe and manually add tickers.
Just be aware that you have survivorship bias and possibly hindsight bias if you are selecting stocks that did well in the past.
For Portfolio holdings, use Portfolio() in a Screener rule. See;
https://www.portfolio123.com/doc/doc_detail.jsp?factor=Portfolio