Also remember you can "transfer" weak duplicate monsters for extra candy. Helps with some of those rarer monsters to get to required number for evolution.
Don't bother "leveling up" any monsters unless you need to give one a little boost for a gym battle.
Also note that "XL" monsters have extra HP and defense, "XS" has extra speed and attack. The curving white bar above each monster shows their "potential": how much they gain upon evolution or leveling up. the farther around this bar is, the more growth potential a monster has.