Here's your answer. http://www.tp-link.com/en/article/?faqid=406

From that, I would interpret that the whole system will drop to the minimum bandwidth, much like other backward-compatible devices which are compatible with lower speeds, unless someone knows otherwise, feel free to correct me. But, unless you're on a gigabit enabled device, the maximum your device will be able to receive is 100mbs anyway, both on your 500mbps and 300mbps adapters. Connect to gigabit with the 500mbps and you'll get the full bandwidth, but not with the 300mbps as they do not have a gigabit port and max out at 100mbps to any ethernet port. Of course sharing the bandwidth between multiple homeplugs to one main modem side homeplug is where you'll see/need the extra bandwidth, and in your case in will drop down to 300mbps as long as that plug is connected. You're only after wireless for basic internet so, the answer to your question is yes it will work and yes, the whole system will be at 300mbps so long as the 300mbps device is connected.

p.s. look for the standards under the spec of the device, if the standards are the same, regardless of the make (e.g. HomplugAV) then you're good to go!

Hope this helps!

i want to configure TL-WR841N wireless router , i have juniper ssg 20 with DHCP configuration .
1-i have one uplink to ssg 20 to wr841n .
2- i want wr841n use ssg20 DHCP Server
3- what WAN connection type required it think dynamic ip mode is batter