When dealing with so many PCs it's a bit too awkward to use formulas to determine reasonable enemies. Don't get hung up on the number, as the system isn't built to handle corner cases like yours anyway (just like it does poorly with high numbers of enemies).
A good rule of thumb is to take a reasonable encounter for their level and double the number of the enemy.
Of course, space issues and Area of Effect spells make this awkward (both ways, as they'll hit all the PCs when enemies fire them.)
Published adventures will be awkward, as you'll have to widen all the corridors & rooms, or add new ones, possibly to create two fronts. (You don't want the extra firepower focused on one defender by default, only be players making a tactical error.)
Add more doors in and out of rooms, widen doorways, and so forth.
This allows for a less clogged battlefield and greater participation.
Having run 6+ player games where Leadership was allowed, trust me on this.
Have a blast & Cheers.
If you can't make the space to fight all at once there are other tricks:
1. Waves of enemies, perhaps through Summoning, not just "more in back"
2. Double h.p. on enemies (In this instance Sleep & Color Spray would have to be adjusted, perhaps to two saves or to somehow the monster becomes half as effective)
3. Respawning (usually Undead that rise back up, or creatures that birth out of other creatures' corpses. Most would have to be invented)
4. Flying, increases space :)
5. Swarms, tiny creatures, creatures with "swarming" (like Morlocks), share space
6. Reach weapons/missile weapons, allows more to participate
7. Combat Reflexes, adds extra attacks, but by its nature it spreads them equally (and to people who can choose to risk it or not)
8. There are some spells & class powers (Oracle Mysteries mainly, but Wizard too) that target adjacent enemies, often as a swift action. That's another good way to bump the offense without the danger of focused fire on one PC. (Defenses that hurt the attackers, like thorns, are another way to make up for the action economy.)
The same issues you face when getting the 9 PCs all able to contribute are what you face with these baddies. See if there are choices your players are making to maximize participation & see if they can be applied to your monsters.
Good luck & Cheers
Sleep and colour spray are based off if hd, not hp
Obviously.
Doubling the h.p. doesn't help toughen against non-h.p. attacks.Hence the need to adjust the rules for those type spells when used against single creatures who "behind the scenes" represent two creatures.
OP:
Adding NPC levels (to monsters with racial HD) is another way to make sturdier opponents without making them that much more dangerous.
Ten people at the table?! First off, no. Five players is my limit. Second off, you could have two gaming groups.
Bringing a 2nd ed player into a group with rookies is good. I believe in the passing on gaming culture to new players.