There are so many good combinations of characters these days that you can't go wrong. Certainly the combination that you have mentioned sounds solid. A few points spring to mind so in no particular order let me do a dump of them:
Mage/SK is a great PL combo. I use a grouped 78 SK with a 85 Mage (plus 2 x healer mercs) in Grieg's to PL a third account. Third account is ungrouped and whilst SK does AE aggro and mage does 3 x DS (long duration DS, 3 hit +650 damage DS, and 10 min refresh 30 hit uber DS) on SK. The third account does AoE (or cycles through targets and kicks them if need be) to get the full XP. I've PL'd a couple of characters to Max AA for their level (up to 62 or so - 600 - 650 AA) this way, so it's a very effective team.
Tank classes take a huge amount of work to get to a useful (tanking) point for higher level zones. an 85 mage pet with decent AA will outtank an SK at 3500AC 800 AA (I use that as an example because that's the point my main SK character is at - so that's from personal experience). Mages get a couple of spells that allow a decent portion of incoming blows to be entirely blocked - which gives them a huge advantage over player characters until the PC is decently geared AA'd. Having said that SK's rock - lots of fun to play when you have some well spent AA.
Necro's are a great class. Versatile, huge DPS available. I played one as a main for four or five years and love them to bits, but I do have to say that if you want to get the most out of them then they need a lot of attention during fights. I'm not saying that you shouldn't include one but be aware that you have to drop on a lot of DoT's. Feign your character after every two or three to start off with to avoid pulling aggro, keep track of which DoTs have dropped (all different durations) and refresh them (remembering to keep the DoT, DoT,. FD cycle going). That's not too arduous but when you are dividing your attention between three characters, trying to back up your merc clerics (a blessing for boxers), keeping aggro and position with your SK, then it can start to tax your brain - smile. I've boxed combinations like that and had fun, but at the end of the evening I tend to feel wiped due to total focus it requires. Currently I box a Chanter (usually with charm pet) on account one, Mage on account two, and Wizard/Bard/SK/Ranger on account three depending on what I need at any time. Account one with a charm pet gets 80% focus and accounts 2/3 just the odd recast/melody change - that works well.
Consider getting a merc tank for one of your box set. For my character combinations pulling is easy, but if you are in tight quarters then pulling with one character and switching to my tank/locking aggro can take a fraction of a second too long - a merc tank will grab aggro as soon as the mob hits the camp and give you plenty of time to control the situation. Not so much of an issue if you use an SK as your puller and MT, but worth bearing in mind.
So here are my thoughts on new characters to level up.
Wizzie rather than Necro - transport for your box set is great, especially if you have mage too. Wizzie gets you to/near the target zone, mage runs forward and COTHs other two to the zoneline/camp. Good DPS, and you can just drop the odd DD rather than spend time on managing DoTs. You do need to cast the odd aggro reduction between DDs but that's no harder than a necro FDing. Instant AA evac as needed and snares to round off the package. Useful class.
Mage on second account. Great tank (I've farmed City of Dranik named in SoD with mine though that's at the top edge of my ability), great DPS, good utility for transport and pulling (COTH).
Shaman on your third account - I'm assuming that one is non-negotiable. The only problem with this setup is the lack of a puller (chanter/monk/SK/bard) - so you could switch out the Wizzie with a Bard (or SK if that takes your fancy). Much reduced DPS but such a great utility class and the melody command to automatically switch between songs makes them great for huge boosts to AC, Damage Shield, spell focus, mana regen etc. If it can be pulled, a bard can do it
Hopefully there is something in that which is of interest. Whatever you choose though I wish you the best of luck.