Without having tried the recipe I can only share a few thoughts.
Firstly, if the caramel sauce you have made according to your recipe is too firm, can you soften it by heating it gently without getting it too hot for the whipped cream? Or is it firm even when it's warm? Maybe it's just too cold.
Secondly, proportions for eggs / gelatine will vary depending on the texture you want. Any calculations would be done according to the volume of the caramel sauce, not just the sugar. It would depend on what your preference is. If you want a very light, easily pipeable mousse then you may only need whipped cream. If you want the mousse to have a traditional velvet texture then whipped cream & egg yolks may be enough, maybe 1 yolk for every 50-70mls of sauce? Gelatine would give a firmer texture again, depending on how much you use. I don't have any ratios or guidelines for eggs and gelatine in mousse, sorry. If you decided to use egg yolks and wanted some shelf-life, I'd want to heat the yolks- whisk them with sugar until thick and then pour in the caramel sauce, like a custard.
Thirdly, if you're prepared to try different recipes or create your own, then my basic ratio for a caramel sauce is 2:2:1 sugar:cream:milk. This would equate to 200g sugar, which I would caramelise, then I would add 200mls cream mixed with 100mls milk. As I outlined above, once the sauce has cooled I would then add whipped cream, gelatine and egg yolks depending on what I wanted. I find caramel to be a fairly delicate flavour, so I wouldn't be adding much whipped cream at all. Whipped egg whites would make it lighter and airier.
Finally, there's an alternative recipe for caramel mousse as part of this dessert
(you can ignore the other elements). I've made it a few times and find the caramel flavour to be very delicate, I'd probably use less whipped cream than the recipe asks for.
Hope this helps.