As far as how the game mechanics are for the added summons, I remember Skabot on another server has a character "Mort Tallytee" who was some strange death domain cleric/wizard. He could summon his familiar, an undead skeleton and his shadow companion all at once, which made for an interesting swarm. I'm pretty sure that a Druid/wizard could have an animal companion and a familiar and their choice of either a druid or wizard standard summons, but not both. The familiar/companion/shadow creating feats appear to operate independantly of eachother, all three of them growing in power based on the character's mix of levels. The casted spell summons are unfortunately limited to 1 at a time regardless of type. So you can never have a mithral golem summon creature 9 plus an animate dead skeleton or a helmed horror or any combo like that.
I miss Mort, he was the quintessential "true necromancer". He'd have 2 undead that could be healed by negative energy ray out, plus some kind of hellhound summons all at once. The downside was of course that other than the base skeleton which has a cap, the other two were a bit stunted by virtue of his cross-class. Having a character at 10th level with 5th level summons looks nice, but they die pretty easily.
One of my biggest beefs with NWN's engine is how they manage summons. I hate how they hardcode the "only one summons at a time" template, especially regarding undead, which are created from dead corpses and animated, not pulled out of thin air in PnP. Thanks to Bioware, you never can have that necromancer with an army of skeletons, only the villains can. It's kind of silly to see somebody like epic Rufus limited to a single skeleton and a fimiliar, while Joe the 12th level necromancer villain plagues the land with his army of 500 skeletons and 200 zombies. Bioware totally butchered one of my favorite PnP spells of all time... Elemental Swarm! Now it's more like, elemental single-file DMV line.
As for the hard limit of paladins, druids and monks from crossclassing, I've never been a fan of that, and eagerly look forward to the new skillbased system of the MMORPG which will negate all of that precident. If being a druid is a way of life rather than a skillset or a job, then there is no difference in my mind of a pure druid who obeys the codes and restrictions, or a fighter/druid who does the same, yet simply knows how to fight a little better with some added feats. A barbarian/druid would be a shoe-in! Turn into a feral animal and rage. A rogue/druid seems like a natural progression to me for many characters for whom the focus is stealth and blending into the woods. I used to have a character on another server called "Master of Squirrels" who was a rogue/druid, and the combo made him FAR more squirrely than a meager full druid who could not hide well and was too quick to dodge a fireball. A NG elven bard/druid could definitely work, I imagine the Disney'esque fair maiden of the woods, singing with the birds not breaking any codes or oaths or immersion. Thanks to the hardline multiclass rule, it's exceedingly difficult to create VERY typical and logical combinations, like a monk who studied animal fighting styles for inspiration, a paladin that focused more on prayer and faith, a wujen style mystic monk who shot fireballs from his fists, etc. What's even stranger is how Lucinda's Paladin order seems to allow paladin/wizards to crossclass freely, yet a paladin/sorceror is an anomaly. I would think if anything the paladin/sorceror would be MORE logical... somebody has faith in lucinda because she touched him with magic, blessed by grace rather than well studied, that sort of thing. Which again is why I'm so eager to see the next incarnation of Layo, when character concepts are no longer meta-policed by Bioware's hard class limitations. In terms of what it means to be a druid, I think lifestyle and conviction far outweigh adherance to a vanilla skillset.
If Grovel could be a druid/rogue sometime before epic levels without having to create a river of paperwork and cdq requirements, he'd go for that, definitely. His personality definitely supports the sneaking, bluffing and the conniving character qualities of a rogue, and I fail to see how the great oak is going to smite him OOCly for daring to hmmm, use foliage as cover. Maybe the RP expectation is for him to chain himself to the tree and protect it from axes with his tiny NE body? The only thing keeping him from doing that currently is a booming OOC voice from the heavens that would scream "Naughty Grovel! you have learned how to sneak in the woods, and now I take your spells!" *ZAP!* "Grovel is sorry! He only wanted to disarm the bear traps! forgive me big tree!" heheheh.
I wish for druids the restriction of going back and forth between multiclasses without needing to do a CDQ just to learn spells again should be laxed a bit for at least the racial Favored Classes. If a halfling or goblin druid wants to be sneaky, that's in his nature, same with orc and giantkin barbarian druids, or elven wizard druids, dwarven druid fighters, etc. For the Great Oak to disallow this by some mystical mechanism shows that he denies them their own Nature. The philosophical implications behind that are astounding. There should still be CDT support and maybe even a CDQ for taking the secondary class of course, and they should be expected to maintain their oaths and armor restrictions, but there shouldn't be such a steep relearning penalty for doing that which comes natural to a druid.
And an aside note: Vorax should be nicer to his Paladin/Fighters too. Many of them must grumble about the oath to never specialize in the waraxe without being cast out of the order! And I doubt he or Toran would be anything but pleased if a Dwarven Defender joined the ranks. Vorax might even do a little happy dance and spill some ale if he saw one praying!
Soon though, all of these Bioware restrictions that don't make sense will be abated by the open skill system. And then it will be we who do the happy dance! Eagerly anticipating any Dev Log reports in that department.