My philosophy regarding hybrids was if I could make 2 level 40 creatures, it was time to make a hybrid from one of them.
Regarding their use in tournaments, the Common hybrids are too weak for Dominator League in standard tournaments, even at level 40. The Rare hybrids are very useful. Other than Diplotator, I routinely use several copies of the level 30 rare hybrids in my tournament teams. Diplotator needs to be level 40. Super-rare hybrids can be useful at level 20.
The game loves to have Common, Rare, and Super-rare only events were you need multiple copies of your top creatures, enough for five rounds if you want to avoid cooldowns. Additionally, we have had a tournament in the past year that required Rare only creatures and one that required Rare and Common only creatures. In both of these cases the hybrids ruled the day. The Common hybrids needed to be level 30, while level 20 Rares worked very nicely.
To allow me to perform better in these events, I have made three level 30s to accompany my level 40 for the Common and Rare hybrids. I am working on a similar program with my super-rare hybrids.
For typical tournament runs, I start with level 40 Legendary and level 30 tournament creatures and work my way up to level 20 VIP creatures, although I acknowledge that there is a greater chance that my beginning teams may not be successful. It depends on the opponent draw. Any hybrids that fit in with this get used.
Hope this helps