Looking for a good secondary or tertiary can be a tough decision. Most people I've talked about this with are trying to cover holes in the match ups.
I approach it a little differently myself. I'm spread a bit thin as I have co-mains and back up plans in various weight classes.
Duck Hunt is the captain of my range game and my top pick on For Glory. However, Cloud and Ganondorf are my co-mains. When I'm not confident in those three for whatever reason, I turn to Rosalina or Roy.
I practice most with Duck Hunt but from time to time I have hard matches which call for one of my other four. Who I call on depends on what gave my Dog problems. In a sense Duck Hunt is my measuring stick.
Rush down: This has been the most common head ache for me. It bogs down my can tactics and generally throws me off. If the person is super aggressive non stop I will counter pick Ganondorf. If they pressure with perfect spacing with someone like Marth, ZSS, or Diddy I switch to Cloud.
If they use Samus or Link I always counter pick Rosalina. I loathe Samus and Link against Duck Hunt.
I pick Roy when the person likes to hit and run and somehow still avoids Duck Hunt's zoning options. Roy flows well against hit and run.
There are a few match ups I stick with Duck Hunt even if I lost. Ryu is the main one. I have fought various degrees of talented Ryu players (not Trella, though) and I love Duck Hunt's options.
My suggestion is to have your back up based on why you lost rather than the character you faced if you can. It ticks me off that there are characters who dictate me NOT picking Duck Hunt. Maybe one day Link and Samus won't give me fits.