I've done the same in the past - the only issue I had was that whilst the basic phone functionality works, the extended functions (intercom and answerphone, IIRC - maybe caller ID? It's a few years ago) were manufacturer specific. So for a phone to just answer and make calls it is fine.
You should be able to put five or six handsets onto the base, so you can use both of your working old ones. If there any problems, you can always just plug the base unit for your old phones in to the phone line alongside the new.