Question about string gauges.
 

I have a 7 string on which I really like to experiment on.
Low to high:
I usually keep it in C# F# B E G# C# C# and sometimes I tune it to B F# B E G# C# C#
for a drop B tuning.
Now I also found out that F# F# B E G# C# C# is pretty cool.
The strings I use are Ernie Balls Regular 7 slinky. (010 .013 .017 .026 .036 .046 .056)
Now I plan on using the Ernie Ball 7-string Power Slinky (.011 .014 .018p .028 .038 .048 .058) for better stability in the drop B and F# F# B E G# C# C# tuning.

Am I in the zone of snapping the lowest string while tuning to C# on the seven? How much will it affect the sound while I am up there and will the strings become too stiff?

This is my guitar: http://www.ibanez.com/ElectricGuitars/model-APEX2

Going from a 56 to a 58 shouldn't make much difference. I personally wouldn't go thicker than a 52 for that C# on a 25.5" scale, but that's just me, and you seem like you're fine with the current 56 for it.

Well, I can experiment even more going to a 58. Why wouldn`t you go for more than 52? :)

Just personal preference is all. I'm not a fan of that tight of a string, but I'm sure you'd be fine going up .002 in size.