... if YN-568 has only an optical receiver, can I trigger it firing the YN-560 through the YN-560TX transmitter mounted on my camera? Does this makes sense?
You can "daisy-chain" the optical triggering by setting the YN-568EX into S1/S2 mode, and then using the flash burst from the YN-560III/IV to trip it, but this will have all the limitations on range and line-of-sight of optical triggering. And, of course, this type of triggering is manual-only (no TTL/HSS/remote power/zoom control).
If this is not doable, if I buy a YongNuo RF-602RX Wireless Flash Receiver for YN-568EX will this work?
Yes, but you'd have to put the YN-560 gear into RF-602 mode; it defaults to RF-603 mode. And the RF-602 units are a pain to use (once you put a flash on top of the receiver, it becomes very hard to reach the on/off switch on the top and the Rx/Tx units have no locking rings on the feet, which means putting a flash on top of one can be unstable--especially if you plan on using the passthrough shoe to mount a speedlight on-camera on top of one).
It may be better to get an RF-603II or RF-605 transceiver instead of the RF-602-RX. These have the on/off switch on the side, instead of the top and they have locking rings--well worth the extra bump in price. The RF-605 will let you have group on/off control. But neither will let you power-control the YN-568EX.
If you want that, then you'd have to get YN-622 triggers, but then you wouldn't be able to remotely power/zoom-control the YN-560III/IV you have (unless you put the YN-622-TX into Rf-603 mode, at which point you have the same setup as with the YN-560-TX and you can't remotely power/zoom control the 568...). You could stack a YN-560-TX/RF-603II/RF-605 on top of a YN-622 receiver, but...
This is why the Godox system may be better to start out with than Yongnuo if you want to mix manual and TTL/HSS flashes in your set-up and/or advance beyond speedlights to bigger flashes. I tend to recommend starting out with a TT600 and X1T or XPro transmitter. Only costs a little more than going with a YN-560III/IV/-660 and a YN-560-TX, and if you want to add a TTL/HSS flash to the mix, the TT685 has an X transceiver built in, and the X1T/XPro would give you remote power/HSS control over both flashes, and TTL control over the TT685. Zoom control's more limited than with Yongnuo, though.