Well because the tension in the strings isn't that great when down to C you neck shouldn't warp.
I got a floating trem and I find that for E I can use 042-009 and they seem about right, for Eb I can use 046-010 and they seem about right and for D I can use 052-011 and that seems abiut right. So guess for C you are probably looking for the bottom string to be around 058-062??? I'm not really familiar with such heavy strings so I can't really help that much, but C is only half a step above B... so it's probably just as easy to go out and buy a pack of strings for those 7 string guitars and then just chuck the 009 or whatever the 1st string is away...