Just to clarify, I'm running 5E. How much of that advice is still applicable? I'm not sure how much has changed between editions. Oh, and if it helps, the party comp is one rogue, one monk, one paladin, and one sorcerer.
In 5e, rogues have very few limitations on sneak attack. It works on any creature, and to use it they either need advantage on the attack roll or they need an ally right next to the creature when they make the attack (the rogue can even do this from ranged). About the only limitation it has that the 3.5 rogue didn't is that they can only use their sneak attack once per turn (but not per round, so it can be used if they hit with an opportunity attack).
Skeletons are much easier to deal with, since bludgeoning damage isn't a requirement to hurt them, though it can be helpful. As I mentioned above, rogues can still sneak attack skeletons as well as anything else, and clerics are maybe not as spectacular against undead since healing magic no longer has any effect on them, but they can still turn them away, and they have probably the best access to spells that do radiant damage, which pretty much nothing has resistance other than things like angels, and nothing has immunity to it. Also, paladin's smite ability does more damage to undead, though this would probably be overkill against a normal skeleton. And monks bare-fisted attacks do bludgeoning damage, and they get to do multiple unarmed attacks per turn, so they can do pretty well against skeletons.
As for general encounter building, the DMG has a pretty effective guide for making encounters of a proper challenge level, though, depending on how competent or incompetent your group is, Your Mileage May Vary on how much more difficult or easy you want to make it from default. But just remember to budget enough xp for the level and number of players you have (the encounter difficulty chart is set per character) and factor in the difficulty multiplier per number of monsters you're using (this isn't supposed to increase the amount of xp you give for defeating the monsters, it's just to let you know how much more difficult a fight will be when you add more monsters.) If you don't want to calculate out encounters yourself,
this is a very helpful site.
Beyond that, some extra things to be aware of when making encounters: Look at the stat-blocks of monsters you plan to use. Probably the most important thing to look out for is damage resistances and immunities. If your players don't have magic weapons, fights against things with damage resistance to non-magical weapons will likely be harder, and fights against creatures with outright immunity to them will likely be all but impossible (though things with damage immunity are probably above your xp budget if you're at running low levels). But also some monsters can have scary abilities, and one you definitely want to watch out for is the intellect devourer, which can easily destroy characters without much Intelligence, and was probably more meant to pad out high-level encounters or to be used as foreshadowing for such.
Also, to avoid making things too easy, try not to group up a lot of low hitpoint creatures together too much, or spellcasters with AoE's will absolutely destroy your encounters. And if players have magic items with a constant attack, damage, AC or saving throw boost, you can probably stand to throw a bit tougher encounters at them. Not that you necessarily need to, all challenges don't necessarily need to be level appropriate either way. And, if you have one character that's stronger than the others, it's usually a better idea to attack them where they're weakest rather than ramping up the total difficulty to match their strength (which will probably just have the effect of leaving other characters behind).
PREEDIT: NINJA'd A BIT