This is probably a silly question, but is there any good reason why I shouldn’t use a signal follower and gainer together as a compressor? I’ve had a bash at dialling one in to sound okay, I think - I just have a feeling like I must be doing something illegal or immoral here! 
Maybe because Renoise already has a native compressor? Not sure, someone please dispel my paranoia or tell me why I’m making a mistake!
There really is no “legal” way of using anything. If it works and rings nice to your ears, tools and techniques are of little to no importance.
I usually combine signal follower with a 5 band EQ instead of compressors, to just sidechain kicks and bass but attenuate only low frequencies
Well, it will work, but it will work a bit differently depending on the material. The stock renoise compressor will handle very fast attack settings differently, because it modulates the gain at the audio rate, and the gain device operates at a lower rate. In practice this means that the gain device, when modulated will create distortions that are harmonically unrelated to the input.
The settings here are attack=2ms, release=5ms, which is quite fast. If you’re using the signal follower for sidechaining bass, then this will likely not be a concern, because you’ll have a low frequency input and slower envelope speeds. If you’re trying to achieve some kind of saturating compression, though, it’s probably going to sound better with a proper compressor effect.
