+1 to the advice from @RangerZ and @mk24. First make sure that you can use the LEDE router in the first place. In the US with some of the ISPs, it is sometimes as simple as just rebooting the modem and/or releasing the DHCP IP address before swapping the routers. Other ISPs and other places may operate differently, so it makes sense to check in with them first.
Once you have the basic LEDE setup running as your primary router, you can start to experiment. Keep in mind that if this is your primary router, you should be prepared to be offline from time to time if you mess things up and need to reset and/or if you are making changes that requires restarting.
Your WDR4300 should be able to do everything you need (you may need to run an extroot to give you enough space for everything).
You may not need the WRT54GL at all unless you have a specific reason to use it (such as older b/g devices that you want to keep off the main wifi, maybe?). But yes, leave it as OpenWRT and then you can set it up for a specific use case if you need it in the end.