Actually, it's not quite that simple. What element you make the entire grid makes capturing monsters of the same element easier. Also, you can use one of your monsters in the current battle to try to strengthen your chances or properly pacting. Problem is, I've found that even using monsters of the same element as the one I'm trying to capture doesn't work all that often (it doesn't help that the game doesn't tell you what element the potentially captured monster is). I usually just try to pact with just the grid instead. Also, you don't get moves that allow you freely change the elemental grid quickly till a fair bit into the game. Until then, it's a real crap-shoot whether or not you'll get a random grid that'll allow you to capture monsters.
Also, you generally never really know if the captured monster is even useful till you've done a fair bit of grinding. You can eventually grind any monster up to insane levels, but if their innate moves suck and they look lame (which most of the monsters do) I never feel compelled to do so. By virtue of being monsters, your captured comrades don't really add any personalty to battle (unlike their human counterparts, who can do more than grunt or make that annoying high-pitched whine), so I'd rather not use the monsters at all. I only use them at all because the non-Emil, non-Marta human characters can't level-up or change equipment.
For the first part of your post... as soon as you reach the second town you can buy a grimoire to teach any monster first aid, and undoubtedly, marta knows first aid at that point. That's three characters per battle starting at the second town, that can all use a water element, making the elemental grid water easily. At the beginning of the game, you have like 80% chances of capturing the first evolution monster, regardless of using the right element or having a monster help form the pact. It's not difficult at all to capture monsters at the beginning of the game.
As for not knowing if a monster is good or not... any monster can be just as good as the humans you don't put into the party. Yes generally some are better, but it's not that hard to figure out. A dragon will be a great damage dealer. A ghost will be a good spell caster. In fact, capture a Were Dragon, and it's evolutions are the fastest growing P atk in the game, and catch a ghost, and you have the best A Atk monster in the game. Now there are tons of monsters in the middle, but it's not THAT difficult to figure out. It's mostly common sense. Something like a turtle will have great defense, a wolf will be fast, a bear will be slow but strong... etc.
Now as for what monsters are good... it actually turns out the two you're forced to capture... an Imp and a Wolf, can be two of the better monsters in the game. An imp never evolves, but gets some great skills and artes that make it an awesome spell caster when given spells from grimoires. While a wolf can be one of the fastest and stronger physical attackers in the game. Most people overlook the imp, but most players actually love the wolf evolutions. So even if you don't want to capture anything, using the two starting monsters the entire game will give you some good allies.
And just to add, you never need to actually level monsters. In fact don't use them at all, they're completely optional. They add a lot of customization to the game, but they're not necessary at all. Level 50 is a perfectly adequate level to beat the game at, so having all the ToS 1 guys capped at 50 isn't bad. If you choose not to use something that a game gives you, it's not the developers fault. And don't complain about monsters being difficult.
If anyone who played the first ToS could stand the T---S system, where you have to see if a move is good and choose to keep it or not, try the other one etc. Or the Exsphere system where you have to commit to using an exsphere on someone before being sure it'll give good combinations... I don't see how they can complain about this monster system.
I find the major issue is people expected another great Tales of Symphonia, with the original characters being central to the story. They're usable for the fans, and the name Tales of Symphonia is purely on this game to sell it. The game could've easily been a standalone game, no need for the original cast, or even the same world. They did it purely for the fans to have some throwback, but intended it to be it's own seperate game, where they wanted you to focus on Emil, Marta, and the monster system. But you don't HAVE to.