cello strings

ruby

How old a cello are you talking about?
I have cellos made in 1877, 1960, and 2004 that all can support modern strings of similar tension. When I got the oldest of those cellos in 1949 all my strings were gut-core with a bare-gut A, probably of lower tension than the steel-core modern strings. I used strings of that type until sometime after 1970.
In a concert a couple of weeks ago the cello soloist played an 1854 Vuillaume with modern strings, including a tungsten-wound Spirocore steel C string.  The main consideration is what string tension and what kind of bridge (etc.) bring out the best tonal and playability characteristics of the instrument in question. I don't think instrument fragility is a particuar concern. Most cellists have probably spent thousands of dollars (at today's values and prices) finding the best strings for their instruments. There is actually quite a range of string tension available in modern strings. I