speaking to a cactus nursery chap recently about potting compost, he recommends John Innes No:3 mixed with horticultural grit. Some folk put peat in the mix but reading up the National Cactus Society say folk are not using peat anymore so a 50/50 mix with little or no organic material. Perlite is a common addative. Have a look at my blog 'Visit to Cactus Nursery' & you'll see the cactus nursery plants where I visit. If we are talking about the same plant there is a photo of an elephants foot ( testudinaria elephantipes ) also. Regards your Ponytail looking abit off it's impossible to say why without seeing it. Gently knock it out of it's pot & check the roots, too wet & rotted? root mealy bug or indeed any pest infection? maybe just a little too dry ? However, if you repot in John Innes No3 this is a really good loam based material with enough goodness to help revive any plant if it is starved. You are right to keep them on the dry side all the same, especially from now untill next Spring.

Thanks