For monsters you plan on using in the long-term, it's better to wait until the cap. You're losing out on a lot of stat points changing them early. Obviously, unless you've played before, it's a bit of trial and error, so for now, at your point, the ones that are worth keeping until the end of the game are the Fairy from the forest, Sid, and the little pink Naiad from the trials. Both of them make excellent healers.
Boneheads, which I mentioned up there, are worthwhile later on, as are the Banana dudes that roam the desert.
I hear the Bighorns are good, too, but I haven't tried them, so I cannot confirm the accuracy of the statement.
Really, the rule of thumb is that anything you use works. Some are better than others, but only one or two of the Familiars are entirely unusable. Mitey works for pretty much the entire game, but will fall off in usefulness post-game. The same with the Lemur, but he has it slightly better.
Keep in mind that monsters have different growth rates too. Some have high stats early, but cap earlier. Others start with very low stats, but bloom later, over time.
Er, one last thing. If you want to be really anal, you'll notice that monsters have different signs. Monsters can "double" these signs, giving them a superior growth rate over time. You can tell if it's doubled by the slightly altered icon. There's also a "Planet" icon that is very rare; it gives the monster a slight boost in bonus damage against all of the other icon-types.