Hello,
I recommend you to buy the same model, because this way you can resolve possible negotiation issues, but of course you can buy any other SPA model.
To connect the phones, you will need SIP accounts for each of them, so they can register and make inbound/outbound calls and, yes, you connect them to a router or a switch (e.g. where your current SPA504G phone is connected).
If Telstra is your current phone service provider, keep in mind that your phones might be provisioned and you will need to ask them how to extend your service to more than one device (additional settings or setup might be needed from the Telstra side).
I hope this answers your question.
Kind regards,
Milan Milanov