Depending on your budget, either Mackie 1832s or 1821s would be good. The 1832s have a wider sound, good for electronic music, especially when paired with a set of 1801s. The 1821s are good fill or for smaller events. I would also suggest a pair of 450s for monitors if you are having DJs and/or producers.

a couple of additional recommendations based on experience running a number of mackie configurations.

1) if you have two 1801's, you can get nearly twice the output if you stick them together in the middle right in front of the DJ/performer. in addition to more bass, you'll have a sweetspot of bass through the middle of the dancefloor/room. whereas, when you separate them, the sweetspot has a tendency to be back 10-15 feet from the DJ. not good for those up front. since you have the 1521's you can put those on stands.

2) it's good to have an external EQ to tweak the system. at full volume mackies tend to be harsh in the treble range. I think this has more to do with power and driver efficiency, and that the tweets get louder more easily than woofers which require more power to get louder. I generally cut frequencies 8khz and above to keep the sound from killing people's ears. The other advantage of the EQ is it has gain, so you can boost the signal out of the mixer keeping the mixer in the greens.

3) A compressor/limiter is another good piece of gear to have to safeguard the system from volume spikes and to keep from having to babysit those DJs that like to push the system louder than it can go.

Have fun with the new setup :)