Can I connect to Litecoin, Dash, BitcoinCash, Doge, etc. RPC with Bitcoin RPC client?
Yes, you should be able to use a Bitcoin JSON-RPC client to interact with JSON-RPC servers for other cryptocurrencies if they are using software that is based on Bitcoin Core (which most are). Such JSON-RPC clients and libraries don't really have anything in them that is Bitcoin specific; they just handle the connection to the RPC server for you and return you the JSON formatted result.