Pitching Question

So I tried to use a different pitching strategy for one of my teams. The idea started after one of my relievers pitched in a game and he was supposed to start the following game. I asked about it and was told he wouldn't be fatigued when he did start.

So I made a staff with a bunch of starter/relievers. Every pitching slot like middle relief/closer/setup etc I listed different combinations of those 6 starter/relievers. I have 3 relievers who I haven't listed in any category. Apparently that won't work as my 3 dud relievers, who I was trying never to use, are getting the brunt of my relief innings.

So that 'plan' has wasted me some money and a season of watching my team get pummeled.