Is this possible or will it slide down the cake? I havent used piping jelly before. Is there anything i can mix to it so that it is firmer?
Thanks

Where you live it probably is hot and humid. That probably will effect the consistency of piping gel which could cause it to run. I guess you are just going to have to try it. Here in the states - especially in drier parts - it would not be a problem.
I can't think of anything that will thicken it up. There are different brands - some more thick than others.