If you can "hear" the sub, as opposed to "feeling" it, then different sizes should be discernible to your ears as differences in tone or volume. Therefore, I would prefer to go with identical subs. If you do use non-identical subs, I think that the crossover frequency should be quite low so as to avoid hearing any differences. Keep in mind that the open E string on an electric guitar is somewhere in the mid 30's in terms of Hertz. I wouldn't want to crossover any higher than that.
As far as sub speed goes, my experience is that the higher the crossover frequency, the slower and more sluggish it makes the main speakers sound. How fast a sub is depends on a lot of design aspects in addition to cone size so I wouldn't feel comfortable making any generalizations. Speaker designers can tell you a lot more than I can on this point so I'll defer to them.
As far as sub speed goes, my experience is that the higher the crossover frequency, the slower and more sluggish it makes the main speakers sound. How fast a sub is depends on a lot of design aspects in addition to cone size so I wouldn't feel comfortable making any generalizations. Speaker designers can tell you a lot more than I can on this point so I'll defer to them.