You could be worrying a just a little bit too much about the debuffs. Yeah, they bring down your character's stats some, but if when you take the other difficulties presented by a dungeon into account, you might find that you can soldier through to the end even with diseased/poisoned characters well enough. Paralyzed is of course, more of a problem.

Healing spells can usually make up for HP lost by diseased/poisoned people during rest.

If you want to keep your folks in top shape on the way through, come into a dungeon prepped. For conditions you can't deal with, carry a clutch of potions in your inventory throughout the game, and restock a few as needed at the shops after a job.

Better yet, Body/Mind/Spirit magic have spells that fix almost everything (especially Body). Look at the spell listing for the Self magic schools. Ask questions to yourself about what you're going to need to be able to learn those spells, and make a plan about how/when you're going to be able to pick them up and make them a part of your regular toolkit. Until you can get to that point, think about which dungeons and other threats you'll be able to take on without those spells, and pace yourself accordingly.

I always grab a few Cure Disease and cure Poison potions if I don't yet have the spells. You can pick them up at any alchemist. If you're tackling Medusas without a Cure Paralysis spell or Protection from Magic, then it goes without saying that you should get a potion/scroll or three (maybe an occasional reload).

How much debuff you can just deal with, and how much prevention/cure you need in your back pocket is something of a balancing act. One that is learned.

Back in MM6, medusas could turn characters to stone (for which Stone-to-Flesh was the antidote), while gargoyles (for example) were the ones that could paralyze. Are they really different in MM7?

The Empress Medusa will stone you--- lesser Medusae will paralyze you. Bows are also used by Medusae in MMVII instead of "mental" attacks.

These games are from a different era, a time where it was perfectly alright to make games where you could get your ass whipped if you weren't careful and went straight into an area that never was meant for your level. Opponents in these games did not scale to your level; it was MEANT to be this way. More or less like a pnp rpg, in other words. Actually I miss those times, which died approximately by the time the first Dragon Age game was released back in 2009.

The point in MM6, 7 and 8 are to use your head, and build your characters a little up before venturing into battle with most of the enemies you can find. Talk to poeple, get quests to do - and then move on to another town using either stables or boats, repeat. Along the way you will pick up some experience, use it and the money you get to train and perhaps purchase some useful spells. By the time you are at level 5 or above you can generally start taking on most monsters in the starting areas, but you WILL be smashed if you go far outside town in e.g. Tatalia with such a party.
Keep in mind that doing quests are what really gives experience, so focus on that rather than monster slaying, at least for a while. If a map proves too difficult, run like hell to do what you came for, and return later to deal with its denizens if they prove too strong for you at your current level.