I tend to stay away from stereo tracks. The only times I use stereo is when using overheads from Superior drummer. Everything else is mono. It's just easier and less confusing to work with that way.
Saying that, it means I don't have a lot of experience messing with stereo plugins for that reason so I could be wrong but I believe if you have a stereo track going into a mono plugin then you are going to end up with a mono signal. If you go into a stereo plugin but with a mono track it will technically turn it into stereo at the stage you have the stereo plugin.
For example, mono guitar track into a stereo chorus effect. You will get the widening effect of the chorus but it will still essentially be a mono track but with a slight delay coming out of one speaker making it sound stereo. Then if you were to add a mono plugin after that it would convert it back to mono and you would lose the widening effect.
It can get quite confusing. For example, some EQ's give you the option for stereo where you can add EQ settings to each side separately. This will only work properly with a stereo track.
Simply put, if you have a mono track then stick with mono plugins unless you want to widen it and if you have a stereo track then stick with stereo plugins unless you want to make it mono.
BUT there is an exception though which is probably the one you want to stick with unless you want to add different effects to each side of a stereo track, that is the one you mentioned in your list "Doubler2 Mono/Stereo", this allows you to run a mono or stereo track through the plugin and it won't change that. Meaning you can add an EQ to a stereo track and it will effect both sides the same.
I'm 95% sure this is correct but if I'm wrong then anyone feel free to correct me, haha.