p2p exchange is possible but need heavy nodes storing all blockchains involved
Only the ones you're actually interested into trade.
If you plan to federate through many altcurrencies is probably because you plan to provide a service; if you do plan to offer a service it's perfectly reasonable to you can provide at least 1TB storage as well to use in your network.
The common use is supposed to be sync between three blockchians: the two altcoins and the federative one.
For example, if you're a dogecoin user... you may be interested into trade just between bitcoin and dogecoin because bitcoin serves better (atm) as value storage move (widely supported).