Ships & Modules

Light drones.
No, because they have awful tracking that counteracts that unusually high sig res against smaller/faster targets.

It's quite simple:
Frigates → Light drones.
Cruisers → Medium drones.
Battleships → Heavy Sentry drones.

You can use a drone class one size up, but it's less than optimal. For ship classes that are in-between of the the three (i.e. destroyers and BCs), you can use the larger type of drones without much issue.

Oh, and unless we're talking about sentries, never ever use the Amarr drones — they're horrid. Use thermal drones instead (they have the highest raw damage output, and all EM-weak enemies have thermal as their secondary weakness so you get a lot out of that increased damage as well).

Never not bring 5 small drones with any pve ship.

Light drones are all you need, as others have stated. If your drone skills are good enough and do the right damage type, you can even take out enemy battleships with them. Anything else will have difficulty tracking frigates with their weapon systems.

Never go in to battle without them if you have a drone bay.

On an Abaddon your drone bay should be filled with 5 medium tech II and 5 light tech II drones. The mediums are for npc cruiser and above and the lights for any frigs you did not already kill with your guns.

When you warp into a pocket and target a group, I always shoot the frigs first. The frigs can scram you, web you, or put other ewar on your. Some missions have ewar cruisers or higher, but the frigs will be the only ones that will scram you. Therefore they are priority to snipe before other targets. Generally i progress up in ship class. Frigs then Cruisers then BC then BSs. One exception is where there are BC in the mission along with HAC npcs. I will kill the BCs first. THey are usually pretty squishy but do a lot of dps. The HACs generally don't hurt as much but you can spend a lot of time getting them down.

Heavy drones are basically a pvp weapon. They are for battles where you are on top of your opponent who is in a large ship and you want to do lots of damage up close. Missions are for sentry drones, medium drones, and light drones. Some ships have large enough drone bays (domi) to carry sentry and heavys. But for most situations heavys will drive you nuts and actually lose you dps because they are so damn slow to get to target.

As to em drones. The only ones presently that are worth using are the sentrys. If you have em weak npcs, chances are their thermal resist is second worst for them. Thermal drones do a lot more damage than em drones. So forget praetors, infiltrators, and acolytes, use hammerhead and hobgoblins instead. There is a thread on S&M where many of us were discussing the stupidity of the drones stats and the uselessness of most em drones. It's one of those things CCP seems oblivious to.

Maybe you don't, but I am just assuming you are talking about drones.

While it is a good idea to target paint small targets especially for larger drones it is a very bad idea to web targets for drones.

Larger drones will start chasing fast targets after a while instead trying to orbit them. That way they can hit even much smaller targets. If you web those targets the drones will just keep orbiting and miss all the time.