Considering I've been running mostly single-player campaigns since I was started DMing 14 years ago I feel like I'm in a special position to offer some advice here. The only real problem is that to me single-player games are "normal" and easier to do than games with three, four, or more players. The reason I prefer them is that it allows you to fashion a character's story in much more detail. You have one hero and you don't have to worry about giving everyone else equal time. It allows you much greater focus on the storyline providing something more like
Beowulf than
The Lord of the Rings.
As for your questions:
1) I'm afraid I'm not much help here as I don't generally use pre-generated adventures very often. My advice is to just use one of the general (four-PC) adventure and fill in the rest with NPCs. Make sure, however, that your PCs chosen class is the one that recieves the most focus in the adventure. For instance, if the adventure involves breaking into a notoriously trapped sewer/dungeon area beneath a bustling city and recovering a relic it's probably best that your hero (PC) is a Rogue or trap-inclined Bard and not forest-loving druid with a hawk as a companion.
2) As far as running two or more characters it really depends on the kind of game you have in mind. If it's a mostly hack and slash adventure then it probably doesn't make much of a difference if the player has two character. I find that for story-central campaigns, however, it's generally not such a great idea. There's nothing really more dull than having to converse with yourself, even if you're playing two different characters. Granted there are players who can pull of role-playing two characters or more quite effectively, but they generally tend to favour one and the other becomes more of a sidekick. Just my experince, however. I still recommend you have the other party members be NPCs.
3) Unless your PCs character is a true loner, I recommend giving him some NPC allies to back him up. Even that scimitar-weilding drow guy has pals. They're interesting pals, too, who can hold their own, but rarely outshine the dark elf himself. And, regardless of how good the character is at some aspect of the game, there are going to be things he can't do as well as other characters. So, you can either elect to leave those things out of the game...or give him a party that can back him up.
Here's a long-winded example which you may feel free to skip:
I recently took a break from running a single-player campaign to try one with two players. I hadn't done this in a while and I was a little taxed because both characters wanted to play rangers. I'm not one to get in the way of what players want, but how do you get a forested, ex-noble elven archer and a wise-cracking, vengeful hill-dwelling gnome to band together? It wasn't an easy prospect of course, being that their favoured enemies were different as well. Rather than play favourites I didn't use either in the initial adventure...and neither seemed too upset about it.
The adventure which I used was actually a pre-made adventure called
The Alchimist's Eyrie by Edward Bolme. Anyone whose familiar with the Wizards of the Coast's downloadable adventures has probably heard of it. In the story there's a alchemist working on a cure for a terrible illness when a gang of lycanthropic dwarves attacks his tower. The PCs arrive after the slaughter has happened...and not only have to deal with wererats but the traps of the alchemist's tower.
How to get these PCs together? First, the elf. After returning to the elven outpost that he visits now and again(he's a 6th level character, after all) he finds an elven maid in his favour has fallen quite ill, along with several other elves. He also finds a paladin (NPC) here, a human woman who seems to know more about what's going on than most. They agree to venture together to the alchemist's to see if he might know of a cure of the illness and save the village. Fairly common stuff. The Paladin is actually in the area because of recent rumours of lycanthropes spreading their sickness throughout the area and is well prepared to a put an end to them. She supplies the elf with silver arrows.
As for the gnome, his entry was even more interesting. His entire clan was wiped out by kobolds, so now he hunts them relentless. Long before this time, he used to gather herbs for the alchemist and was on good, if business-like terms with him. Recently approached by a dwarven rogue (npc) who knew of a band of kobolds that had something to do with the death of the gnome ranger's clan. The dwarf wanted their loot, but realized he needed assistance at getting out alive. All went well in slaying the kobolds, except that the chieftain and his bodyguards escaped. Afterwards, the gnome ranger tracked them mercilessly northwards, until his path crossed that of the elven ranger accompanied by the paladin. There they cornered the kobold chieftain...the usually silent dwarf had him pinned to the tree, speaking in a language none of the others knew.
What was found out was this: The dwarf once belonged to a band of brigand dwarves. He was their resident thief, until it was discovered that, thanks to a hat of disguise and cloak the dwarven rogue was actually a duergar. The other dwarves nearly killed him, but he managed to escape. Now he seeks revenge on the gang...the gang which has recently been transformed into wererats. Furthermore, the lone gnomish thief in the dwarven gang happens to be the PC gnomish ranger's brother...who mysteriously disappeared not long before the band of kobolds slaughtered the entire clan...save two.
So, everyone, even the NPCs has a hook into the story. While he NPCs actions don't really outshine the PCs, they do have their own specialities and qualities which make them stand out. My players seem to like it more when the NPCs are fleshed out and not merely hired mercenaries. It makes them seem more realistic and not as easily replaced if something happens to them.
Hope that was of some help and not merely self-indulgent rambling.
