[quote name='io']I'm taking advantage of this game
not having achievements/trophies by not necessarily trying to do everything in the game (though I can't help but try to do every quest and those high level monsters are too tempting).[/quote]
But it does have achievements... Or do you mean ones that aren't tied to an online account that your friends will see

?
[quote name='io']I haven't controlled anyone but Shulk yet but I do have those characters you mention.[/quote]
I suggest you start switching it up now and trying different characters, or else this game will get really boring really fast. Trying different characters definitely helps shake it up.
[quote name='io']I'm probably not far enough along though in that I haven't found any better armor for Melia than what she came with[/quote]
You can use the skill tree links to give her the ability to wear other stuff like medium or heavy armor. Otherwise you don't really find a whole lot for her.
[quote name='io']certainly don't have enough to outfit people like you suggest.[/quote]
They were just suggestions, you don't
need any of the gems I listed. It just make the probability for success that much higher.
[quote name='io']I also had no idea you could make gems that are higher than the stuff you put in. I really have no idea what is going on with gem making and I've only bothered on a few level 1 and 2 things so far. But I need to do something because my entire crystal inventory is full and I keep having to throw things away.[/quote]
Gems can make your life a whole lot easier. For example, if you max out agility on Dunban he's practically invincible because enemies around your level can't hit him. Other gems like spike defense and spike are helpful as well.
Basically the way gem crafting works is at 100% you get a gem at the level of materials you are using. Every percent over 100 and it makes that gem a little bit better. At 200% you hit HEAT, which makes a gem one level above the materials you use. Every percent over 200 and that gem gets better attributes. At 300% you hit MEGA HEAT which means you will get two gems, both of them being one level up from the materials you use, and both gems will have the maximum attributes possible for that gem level. So if you use level I materials, and hit mega heat you get two level II gems with the best attributes a level II gem can have.
Dunban and Reyn are a good pair because they can make percentages go up super fast. Try to max out their affinity (either keep both in your fighting party or use the option to gift extra collectibles from one to the other) so they can get 10-15 crafting turns. You want to use other pairs to make canisters that are between 75 and 99 percent without going over. Then when you get two like that, you start off with ~150% and Dunban/Reyn can get it the rest of the way to mega heat.
[quote name='io']The way I'm playing now I can handle normal monsters at my level but not special ones usually. And I don't even try to go after anything more than a couple levels higher.[/quote]
Are you leveling up your arts for all the characters and using the skill tree links? Nothing at your level should give you any kind of trouble at all. This game seems like it was made to over level anyone who plays it. Especially with all the side quests.
[quote name='io']Speaking of full inventory, my misc items inventory is also full - I mean the stuff you get from monster drops (antenna, parts, etc). Since I never know what someone will want for a quest I have a hard time getting rid of anything, but clearly I need to do so to pick up the better stuff I'm finding now.[/QUOTE]
Sell everything that isn't a specialty item (the ones without the little sparkle or whatever) and doesn't have the quest marker (an exclamation point) on it. I sorted by value and started selling at the low end and work my way up from there.