Posting to watch I have logged soooo many hours in Wizardry 8. It's on my list of desert island games. I mean hell, I named my DF race after Dracon!
As for the multiclassing question I think you keep your stats you just have access to the unique abilities of a lv 1 of what class you switched to while losing those you switched out of. An engineer can switch to any other class and still have the engineering skill to allow them to use gadgets, but they can't use their omnigun until they switch back, and their omnigun also won't get any stronger while they are leveling as the other class.
It seems that if it translates into a skill you can put points into during level up you keep that ability when you are another class, but you would lose things like the lord's regen, the fighter's berserk and the Valkyrie's cheat death if you are not in that specific class. You also can't put points on level up into a skill that class wouldn't normally have. A fighter with engineering for example can use some gadgets, but they can only gain skill with them from use(maybe), not level up points.
Take this with a grain of salt though. I preferred single class so it's primarily secondhand. My group was always Dracon Fighter, Human Lord, Felpurr Ninja, Elf Mage, Elf Cleric, Elf Ranger.
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Also a little tip for you regarding formation. You don't have to have somebody in every formation slot. My party always had the front slot completely empty, with my "Front line guys" (the fighter and the lord) In the left and right spots, my ninja in the middle, and the cleric mage and ranger in the back. Vi and RFS usually chilled at the sides with the beefcakes.
The open space means front row gets skipped, and anything directly in front of the party is in melee range to all 3 middle sections of the formation. The middle is a little safer because single target damage incoming from the front will be spread to both the middle and the sides (as they are also in melee range) and the beefcakes are on the sides as any groups partially wrapping around the party will be able to melee them and only them. Back is protected by being up against a wall in an ideal world, so nothing can get in melee range of them.