If the third party software provide the API it is easy to integrate it with odoo.

Map the field of both the software and using XMLRPC or JSONRPC You can easily integrate it.

First why you need to integrate third party tool for pos(as Odoo itself giving you good solution, you can customise you need more)..

If you need to integrate then it depends on what you want to integrate inventory, accounting ,... ?(and you need to call external query like what we did for ebay import poducts and query when pos order done,....) And you need customisation accordingly. I don't think this is the easiest job..