Suggestions for newbies

I am new to this community and as well as in the game. Just starting out. So wanted to know if it’s ok to train those monsters without any stars for early leagues? Or should I only focus on monsters with stars? Because after Elite league almost every opponent have 1 or 2 Ultra evolved monster(s). I didn’t want to waste gems as I wanred to buy 10 in 1 pack. So I had to go with 3 Epics and the rest is a mixed of 0-3 star monsters. Having hard time in progressing in both offline and online missions because of this and also because if the team cost limit.

You can train 16 monsters at once, I’d prioritize the monsters with 4+ stars, but there are some catchable monsters that can be useful, especially in the early stages of the game.