yes you are right and of course you can cut off a small part from the big board. Just cut it with a sharp knife and wangle it at the cut area.
1) Solder out the IR diode and IR receiver and solder it on the new board
2) Put a connector like this one (link Buchsenleiste)
3) Put a connector like this one she small board (link Stiftleiste)
4) thats it.
You have to cut the "Stiftleiste" and "Buchsenleiste" to lets say 6 pins, the rest you may keep for later use.
For the second daughter board you have to buy sender and receiver as i mentioned in the earlier post. But please try to find out which carrier frequency the phone uses.
Write down how your connector is soldered to keep this in mind for the daughter boards.
1 |o o| 2
3 |o o| 4
5 |o o| 6
3 SFH 5110-36 out
4 SFH 415-U Kathode
5 SFH 415-U Anode
6 NC (not connected)