No, it's totally a good question! When it comes to caps, the individual canon is taken into consideration. It would be unfair to limit it to six characters from ANY fate/ franchise canon when they are all different.
The only caveat is that adaptations of one particular canon would still count as that one canon. So for instance, playing from a fate/ visual novel that was then adapted into an anime series; these would be considered one in the same when it comes to a singular canon since one is just an adaptation of the other. The only other caveat would be that you can only have one version of a character (ie Saber (Artoria Pendragon)) in the game at once, so if someone apps her from fate/Zero, she can't be apped from fate/stay night.
Hopefully this makes sense and sounds fair to you!
no subject
The only caveat is that adaptations of one particular canon would still count as that one canon. So for instance, playing from a fate/ visual novel that was then adapted into an anime series; these would be considered one in the same when it comes to a singular canon since one is just an adaptation of the other. The only other caveat would be that you can only have one version of a character (ie Saber (Artoria Pendragon)) in the game at once, so if someone apps her from fate/Zero, she can't be apped from fate/stay night.
Hopefully this makes sense and sounds fair to you!