If your torrent client is having a negative effect on other traffic then use its bandwidth limiter.

You can also lower how many connections it makes, but I don't know anyone that's had need to change that. Could you show us which client defaults to connecting to hundreds of peers?

My example was to show locally what happens -- the ISP does not have control over how many connections you make. I'm saying that if you have X TCP connections for HTTP and 100X TCP connections for BitTorrent, the HTTP connections will be drowned out. Therefore, when the link at your ISP becomes congested, HTTP will be disproportionately affected.

For the second question, read the section on choking at https://deluge-torrent.org/userguide/bandwidthtweaking/ and Deluge appears to set the maximum number of connections per torrent of 120 with a global max of 250 (though I've seen 500+ in my brief searching, mostly for Transmission and other clients).

I'll admit a lot of my BitTorrent knowledge is dated (having last used it ~15 years ago) but the point remains: ISPs are built for "North-South" traffic, that is: To/From the customer and the networks with the content, not between customers, and certainly not between customers of differing ISPs.