It's a good thing you're starting to try and blend c4d renders in it. The thing is that they just look put there with a low opacity. First you should try using one basic stock or render, or several, into a picture, then add detail in the forms of c4d's that fit. You can see it in several tutorials.
Second, about the sig discussion, you shouldn't "give up on sigs". You should experience in any kind of thing and not just stick to one of them, because that's how you improve. Sigs are good because their small size allows you to experiment a lot of effects crammed in a small space making it look full and complex. It also allows you to have a lot of play with perspective and lighting. Larger pieces are, as their name implies, larger, and much harder to fill while keeping a busy look.
All in all, the least sigs are is excellent practice.
