Yes, I think that trying to keep it simple is a good rule. Ideally you should only use effect that are really doing or adding something to the source sound ( unless done on purpose for sound design & more creative things of course ). Adding another effect in the chain to correct something that isn't sounding right might not be the best solution, instead it's better to check back the source & the settings of the previous effects. You can add another effect in the chain to "control" something, like for example having an EQ before a Compressor and another one after just to fine adjust the compression results, but if it's really sounding bad/worse after the 1st EQ & Compressor, you might want to check your sound source, EQ settings & Compressor settings before trying to add another EQ or effect to get it sounding right.
For the mono thing, it's a tricky question and I think it really comes down to workflow and listening habits as you mix in the end. Some people like to have all their sound source in mono or/and do the all mix in mono before taking care of the stereo image during a later mix-down stage. While it's true that it helps with clarity, it can also brings issues with the low end dueling in the same frequency range, but it also gives you more control to dial in the stereo effects amount and to control the spread & space of the final mix.
Then other people will mix in stereo right from the start ( and I think it's the most common technique used ), because they need to hear how a wider mix will sound in the end. Like you mentioned, It can quickly sound very busy when adding channels to the mix, but again it's a matter of dialing the correct amount and you'll have to go back & forth between listening to different channels, in isolation, against each other and in the full mix context. For example if you start a track by a great sounding pad and a lovely chords progression, you gonna think it's the fundamental element of the mix and start to build on this, but soon the all mix will sound cluttered & dull because that nice pad was taking a lot of space in the mix right from the start, and it's gonna give you a hard time to hear what you need to make the right mixing decisions. So this channel needs to be muted when introducing your bass and lead synth for example, then play the pad against the bass, synth and all track together and you'll be able to tell if this pad was taking too much space in the mix and dial some settings back to make it fit with the other elements. And so on & so forth when adding a new element. So again, it's more a listening & workflow habit or liking here I guess.
And finally many people don't really care about mono, you'll find plenty of commercial mixes that aren't really sounding nice when switched to mono, they might not sound "bad" in mono, but they often sound drastically different and very thin, some sounds might even almost disappear and that's probably because the producer only took care of getting a mix sounding huge & full in stereo and not paying attention to retain some mono-compatibility.
Who's right, who's wrong ?? It really depends of the rules / goals that you decide to set for your own mixes right from the start and the habits / preferences you develop over time. It can also depends of the "target" and destination of the final mix. If you're sure it's only gonna be played on a stereo system, you might not care about mono but if it's gonna be played in a Club with an older mono PA system, you really need to care about the mix sounding good in mono then.
In the end, I think it's a good thing to check your mixes in Mono on a regular basis during mix-down and to try to retain a certain amount of coherence between stereo & mono. Now for a 100% mix in mono workflow at first, it's again about habits & preferences over time, if it makes things a lot more easier for you to listen & mix, then it might be a good workflow, at least experiment with it and it might also help to learn to dial in the correct amount of stereo processing, again, starting with a huge spread sound will make things more difficult.
EDIT : Editing this since something else came to mind later on, which is the notion of "contrast" in a mix. The above is rather theoretical & maybe more on the technical approach but musically speaking in the end and thinking about this notion of "contrast" :
- What's stereo in a Mix if everything is stereo ?
- What's mono in a Mix if everything is mono ?
I think it's important to have a good balance between the two in the end.