If you had a party including 4 or more multi-classed charecters would it have a negative impact on the game? I only ask this as I am planning a party with me as a cleric/mage, another charecter created in multi-player that is a thief/mage, Jaheira (Fighter/Druid) and Minsc (ranger). Would i have problems with this party or with other predominantly Multi-class parties?

i think you should be fine. if you decide to include a single class character, i´d recommend a wizard or, even better, a sorcerer.
i´m currently playing a party of a fmt, jaheira, viconia, aerie, anomen and haer´dalis and it´s one of the easiest games so far since everyone has access to stoneskins apart from viconia (whom i use for ranged support only unless i´m fighting wizards).
you will miss out on the ultimate power trips only high-level mages get but if you can live without the wonderfully smelly cheese of project image and without decimating entire armies within moments using sequencers... go ahead.

Generally Multi-Class characters are weaker than single or dual-class in SoA as they can't reach high levels, which is extremely bad for a mage or cleric due to lack of high level spells. If you have ToB installed however multis are a fine selection as they can really excel with those extra XP points.

If you only have SoA, it's a tossup: Single and Dual-classes can reach the higher-level spells or get godly Saving Throws that make them practically invincible, as well as taking a kit, whereas Multiclassing gives you a more flexible, versatile character and (for some reason) 20-25% more hitpoints.

BUT. If you have ToB, Multiclasses RULE. Not only will they quickly reach the Saving Throws and highest spell levels that were denied them in SoA, but they can choose twice as many High-Level-Abilities as Single- or Dual-classes. My ToB Powergamer Dream Party is as follows:
Half-elven Ranger/Cleric (protagonist)
Elven Fighter/Thief
Half-Elven Fighter/Druid
Elven Fighter/Mage
Half-Elven Cleric/Mage
Elven Mage/Thief
I'm calling them the Suldenesselites. [img]smile.gif[/img]

I’m very disappointed in the multi-class characters. I’m on chapter 7 and Aerie doesn’t have any level 7 spells yet and I added her as early as possible. Takes way to long to level up. At least in SOA it does, which all I have ever played.

It does take a long time to become "God-like" in one class, but Aerie is able to cast both Cleric and Mage spells, which is her advantage over any other NPC. That's the idea of Multi-class; Some disadvantages and some advantages. I personally love them. Especially Cleric/Rangers

An interesting class to try: a DUAL class ranger/CLERIC (dual immediately at the beginning of the game). It is a single class character so that it advances quickly but:
a) it has plenty of proficiences: 8 from the ranger plus next ones from the priest.
b) it is still a ranger so that it can put 3+ in 2weapons and 2+ in other priest weapons.
c) it gets all priest and all druid spells!
d) its stronghold is the ranger's stronghold.
Do not worry about THAC0 - Crom Faeyr in one hand and Scullcrusher or Blackblood in the other will put it all right. The multiclass ranger/cleric will not be better because of low level.