I tried to play MM6, but the combat is REALLY fast-paced, and I find myself dying at every turn. Any way to slow the combat down? Or tips to make it easier?

Fighting melee at the beginning is not the best way to start. Most players get the reward for the quest in the New Sorpigal Inn and head to Ironfist to buy bows. With bows, you get much better results for a low level party. I can generally do quite a few of the maps with just bows early. To slow combat down, you can toggle to 'turn based' using the Enter key.

THANK YOU, Melee actually works pretty well for me, but I can't click that fast XD Plus, my spellcasters die every five seconds. I got a healing spell... Yeah, doesn't work too well.

Use Pause! You can pause combat and have it proceed in a turn-based fashion. This REALLY helps with melee combat. While paused, you cannot move, but you can issue commands to each of your party members in turn, to shoot, attack in melee, cast spells, etc. The monsters then get a turn to attack (and move a short distance, actually), and then it's back to your party again. This makes the strategic use of healing spells and attack spells much easier, and it's also a great way to use some of the more powerful area attack spells, because you can pause the game while there are some enemies in range and nail them with the spell without having to worry about them moving out of the way.
If I remember correctly the pause key is the spacebar and works at any time. I did the majority of combat in paused mode, real-time I found to only be useful when fighting easy enemies after my characters had levelled a bit.

Be aware that you can't dodge projectiles in turn-based mode (unless you're flying - you can still move up and down while paused). You'll sometimes have to use active mode to avoid being shotgunned to death.

Once you've got some bows, it actually becomes pretty easy, even without pause. Just walk backwards away from the attackers, repeatedly pounding on the A key (for attack, they'll use bows if too far, or melee if close enough) and you can dodge incoming attacks by moving left and right. Take out huge groups in fields by doing so. Then walk forward, clean up the corpses and anyone who didn't follow.
The AI can also be exploited: if you're inside, it's often simple to get enemies stuck behind pedestals or tables, and you just machine gun them with arrows :P

Get Ring of Fire and use it up against doors, etc. Double doors and side-opening doors have a visible gap that you can safely kill enemies through.

Rule One: Press Enter when you see monsters. Always. And you'll usually want to stay in turn mode until the monsters are dealt with, unless you need to move around. But even if you do want to move around, you should spend most of your time in turn mode.

Rule Two: Don't try and kill all the monsters at once. Most times you'll find that you can approach that huge hoard of monsters slowly, and let a few of them peel off and run at you. Once you've dealt with them, get a little closer and let the next few run at you. If you accidentally activate more monsters than you can deal with, run away (zigzagging to avoid their range attacks) until some of the monsters give up the chase.

Rule Three: Try somewhere else. If the encounters are too tough for you where you are, perhaps you don't belong there yet? Through most of the game you have plenty of options of places to explore.

Rule Four: Don't fireball at close quarters. But you knew that, right?

Also: You do have a bow for each of your party members, don't you? That turn-based paralysis problem is an issue for the monsters as well!

Once you have both the fly and starburst spells, you can also corral ground-based melee monsters by flying around in a circle above them, then casting starburst (the movement patterns of things like archers don't really support this strategy). Depending on the area, you can end up with a lot of dead bodies after only a few instances of starburst

Also you wouldn't really want to assemble a huge crowd of death-flinging monsters, especially when it's your squishiest party member who is keeping you airborne.

Actually that brings to mind my major beef with the MM6+ combat system: What's the point in having tank characters if you can't put them in front of your casters?

As far as I can recall, the M&M series from MM3 to MM8 never had an option to make party ranks.

True, yes. My mistake. Only in MM1 and 2 was there any concept of armoured PCs protecting squishy PCs by standing in front of them. And even then it was a bit of a crapshoot.

Yes, certainly in six and later, your "squishier" members, as you put it, generally died first; and almost a challange subsequent to the game plot itself consisted in making sure they got about as many experience points as your beefier characters. (I also remember reading somewhere that in six and later games, if XP could not be evenly divided among player characters, then it was distributed in order of left to right, which is the same order of beefiness that you get in the default party, which ran Paladin to sorcerer. Thank god for Alt+click!)

Also, in the 3d world that 3DO introduced, you and your enemies can dodge ranged attacks. This consideration also affects flying around in circles, and makes moving around a good combat tip, at least as far as retreat is concerned (although in this latter example, you should make sure to zigzag and damn the circles).