I can't speak for other coins but Phore staking works just fine for many people from a CPU perspective on a Raspberry Pi. If you were going to do a lot of zerocoin minting (which has nothing to do with staking), I'd suggest something with a little more processing power like an Odroid.

Swap space can take care of the limited RAM to an extent, at the sacrifice of some performance, but again with staking it may or may not be an issue.

Of course you are correct that staking multiple coins will multiply the CPU operations as well as the RAM requirements, and depending on the size of the blockchains and the number of staking inputs for each, it could cause performance issues, one of which might be trouble keeping up with syncing all the blockchains which is critical for successfully staking.

I would also echo that if you could find an old cheap desktop or laptop for ~$100 and let's say 8GB of RAM and install Linux on it, you'd have a pretty solid solution for staking a bunch of coins, with one downside being somewhat higher electricity costs vs. a Raspberry Pi.