Ok, Turanil-- lighten up.

It's just a game, and you're the GM, after all-- you can do whatever you want.
Here's a little secret: You don't
have to be
fair. Well, you do-- you need to be impartial, of course-- but it's OK if your monsters and villains can do things that your PCs can't.
The heroes' lot in life is to struggle against the odds.
This is a rewarding gaming experience.
Conan never cast a spell, and he ran up against countless supernatural foes. I never once heard him complain, "How come Thulsa Doom can do that and I can't?"
Turanil said:
I also heard, and think agree, that d20 modern classes would appear weaker than D&D classes if one would use both in the same game. Since Grim Tales classes are extrapolated from d20 modern, are D&D classes more powerful than Grim Tales classes?
I disagree. You've already mentioned action points and defense bonuses, which make a difference, but are not the main difference, in my opinion.
The main advantage of a Modern/GT style character is
customization. When a GT character decides he wants to be good at something, he will be very, very good at it.
A talent
within your specialty every 2nd level and a feat
within your specialty every other level, plus all the feats that a D&D character normally gets? You can pick and choose and min/max a very effective character. (I'm talking primarily now about non-spellcasters-- spellcasting changes things quite a bit.)
For the most simple comparison, put a Fighter side by side with a Strong Hero.
Same BAB.
Same skill points.
Both get a Feat of their choosing every 3rd character level.
Fighter has slightly better HD (d10 vs d8).
Fighter has slightly better Saves.
Fighter gets a bonus feat every 2nd level.
Strong hero gets a bonus feat every 2nd level.
Here's where they diverge:
The Strong hero gets a Talent every odd level!
Being able to pick and choose the abilities you want to tweak out your character is very, very effective.
Actually, I think the rogue is the hardest non-spellcasting class to keep pace with. Rogues enjoy a lot of class abilities that scale very quickly and can outpace a GT character. Unless the GT "rogue" is willing to forgo some of his abilities to specialize, I think the rogue will be more broadly useful.
Of course, there is no point for me in allowing D&D (non-spellcasting) classes in a Grim Tales games.
But you could... You won't break anything. If you want to use a published adventure, say, that includes an 8th level barbarian-- why exactly couldn't you just do it? There's nothing magical about the fact that he's called a barbarian (and not, for example, a Strong/Tough/Fast hero); at the end of the day he boils down to a bunch of numbers and mechanics, and you'll find those numbers and mechanics compatible with everything your Grim Tales PCs can do.
Wouldn't it be strange to have characters who don't get Action Points and Defense while other get it?
No. Why? If action points were granted by a luckstone and defense was an unnamed bonus granted by an ioun stone, would that be
strange?
Another thing: I am considering buying Serpent Kingdom, which fits pretty well with a portion of my Grim Tales homebrew setting. However, not only several NPCs vilains depicted do use D&D classes, but they also cast spells in the D&D way. I don't want to buy a book if it's to change everything, and I don't want to have monsters cast spells as in D&D while PCs do it Grim Tales. Even if in the end, one system would not prove better than the other, I do find it non coherent within the world.
First, let's separate spellcasting from spell-like abilities. There's no reason that creatures with spell-like abilities wouldn't carry on using them as they always have, side by side with characters who may or may not have spellcasting ability, be it D&D spellcasting or GT spellcasting. This disparity already exists even in standard D&D. Nobody ever asks why the efreeti can cast
wall of fire at will, but the PCs have to cast
wall of fire as a spell.
If you're playing D&D and none of your players chooses to play a spellcaster, does that suddenly mean that all of your monsters have to suddenly forget how to use their spell-like abilities and spells?
So if you can separate that in your mind, why can't you also allow spellcasting villains that can actually cast spells differently from your PCs? Why couldn't the villain be a sorcerer, or a wizard, learning and casting spells using knowledge that is unattainable by your PCs? (Think Thulsa Doom again, here...)
So what are your opinion and suggestion about other classes, and monsters with spell-like abilities as well as class spellcasting, in a Grim Tales games? Any easy method of houseruling that?
Very easy: Just allow it. Game on!