DC1A is awesome yes.
Grab any other Klanghelm freebies too (IVGI saturation, and MJUC Jr / VariMu compressor).
If you lik em, they're all inspired by their "full" plugins, which are really affordables.
The Rocket has a threshold control so you don't really need input gain.
Just set the threshold to your liking, then compensate with the output gain.
Don't forget to switch on "decadence" on the bottom left, it activate oversampling and just sounds better.
If you want to try other comps, I also like a lot Stilwell MajorTom. It's kind of a dbx160 inspired comp (amazing to make a snare snap).
About ProMB, yes it's awesome and it does a lot more things than C4/C6... but it's way far from cheap or free !
Check BlackFriday deals that are coming (Waves one starts in 24h).