Hello I'm flute and guitar player. I do want to play the clarinet. On to my question. How tight can I tune a set of cello strings above standard? I heard cello strings are strong. How tight can cello strings go without breakage?

When you say you're a guitar player, presumably you don't mean classical guitar, otherwise you'd know to be more concerned about damage to the instrument than to the strings. (A classical guitar's strings should never be tuned any amount above concert tuning because that puts too much stress on the body of the instrument and can cause it to warp.) The term for unusual tuning for bowed string instruments, by the way, is scordatura.

....which is not to suggest that cellists happily break their strings. Good cello strings are expensive.

I want to know maximum tension each cello string can handle.

The absolute maximum tension would depend on the particular kind of string (gauge, brand, etc.). Pieces that call for tuning down are more common than those that call for tuning up, but pieces that call for tuning up do exist. I don't know of any that call for tuning any string up more than a semitone.

I am a bassist & occasionally tried some other instruments like Sarod, Violin, Upright bass etc. I have tried some customization on my cello as I’ve installed frets as I am comfortable playing vibrato technique on frets rather than detache bowing. Basically intonation crisis or tonal geography on fretless instruments can be overcome by practice & paying attention & time but you know, playing comfort is a different thing. As a bassist or a guitarist (that I have been playing for 23 years) I am using certain techniques & moves with which I am really comfortable & I am eager to get that comfort on trying any instrument that I get & I don’t want to be frustrated after trying. Yes, those frets helped me. I have tuned my strings as G,D,A,E instead of A,D,G,C usual cello tune as I am a Bassist & I am comfortable moving on that tune & geography. Well, D remains same, so no problem with that. A drops down to G, G goes 2 notes higher & the challenging part is C goes E. Now, no forum could solve my problem when I asked. I thought the string tension will be so high that the string won’t last or the fingerboard might develop torsion as it has no truss rod inside. So, I tried a jazz guitar flat wound D string & tuned into E. Trust me, it sounds horrible while bowing on it. So, I tuned the C string up to D, waited for a couple of days, tuned 1 note higher, waited again then its E now. It is smooth, creating no problem at all! Planning to buy 1 set Presto black nylon wound string, I hope the result will go even better. The only thing that you should care is the tune goes up & down for the respective strings & you should vary you pressure on bow accordingly. Well, it’s a matter of practice. You may try it as well. You may watch the video of my playing on my customized cello on a casual jam with a violin, acoustic guitar & a fretless bass. You may search youtube with the track title - Blindfold Birds' Ballad on the youtube channel - ShironamhinTV . Here is the link. Some more jammimg videos & demonstration on customizing coming up. Please subscribe & leave your feedback if it helps.
http://www.youtube.com/watch?v=TVjuievNVdg