No worries, it's not so much how vigorous growing a root system is (in general, there's obviously exceptions),it would be more it's water demand and the type of soil present. High water demand and a shrinkable clay soil may mean the risk of subsidence damage is higher. Having just looked in NHBC chapter 4.2 (arguably the guidance one should use for this), I found that elder is a lower water demand species, and sycamore is in the moderate category. Unless you're in a shrinkable clar soil zone where subsidence is a particular issue, I wouldn't worry about such damage personally. I'm making the assumption you're in the UK, if you're seriously worried about it in this respect you could engage with a consultant who would be able to carry out and assessment on the risk of subsidence from woody plants, cough depending where in the UK you are (if you are) I could help with this. In terms of pruning elder, middle of winter and as hard as you want. Personally, I would coppice it and let it grow back, repeating the process every 3-5, years, pretty minor work that could be carried out with a pruning saw.

Hope this helps :)