1) Try and use EQ's for cuts - if you're making frequencies louder you'll think its better regardless. Though adding can be useful for certain effects.
Here's some useful EQ cuts.
Cutting below the lowest note of every instrument - helps remove noise, mud, general shit. All of a sudden you've more headroom. Hurray!
Sometimes you'll need to cut a little of the bass out of the kick or the bass line so everything can be heard - though a common trend in pop and dance is to use sidechaining so that their volumes change in parallel. But try EQ'ing before that, alot of music doesn't really do the "pumping beat" thing.
I'd advise the same for any melody instruments, cut some bands so that certain instruments can come through. If it sounds fine to cut from 1-3Khz from the voice when the high guitar comes in but sounds bad once it leaves then automate it! No one says that your volumes, frequency content etc. have to remain static.
2)Compression - more is less. Again just because it's louder doesn't make it better. High ratios can cause very fatiguing effects. Compression also naturaly effects the transients and frequency content of the signal, so be aware that you can destroy a good sound with compression pretty easily. Especialy since you're using synth sounds (which by nature are synthetic and don't evolve nearly as complexly over time as acoustic sounds since you have to conciously induce these changes) and samples ( which presumably already underwent compression) I wouldn't be trying to squash them if I were you.
Limiters are useful for when something was recorded badly and you need it to stop. fucking. clipping. But honestly lowering the threshold, ratio and using a softer knee on your compressor can give you the same effect without ruining your signal. In your case it'd also be useful if you're using high feedback (100%+) on certain effects or generate feedback loops.
Parallel compression is a nice sound, sometimes. Set up a send with a compressor, route your rhythm guitar, bass and drums (maybe avoid too much hat and cymbals though, or low pass the bus). Same for synths - send your "rhythm" tracks to the compressor.
Be aware of your attack and release times! A very fast attack will kill transients as it'll squish everything, a slow attack will allow transients through. Release times will effect how long it takes the compressor to stop compressor after the signal falls below the threshold level - long times can bring out room noise in recordings but can add abit of tail to overly stacatto sounds.