The general answer is Yes - all the HomePlug AV standards are compatible, all the way from 200 to 2000 series. Slower connections will eat into the bandwidth of the faster adapters obviously.

I had 3 AV500 which were connected and working well.
When I replaced 2 of them with a PA7010 kit, the new ones connected and work great, but the old AV500 won't connect anymore although I followed the procedure to add an adapter to existing network.
I found a workaround by adding back one of the old AV500 into a different outlet and it connected right away with the other AV500.

So they're still running on the old network key. Since the TP-Link kits don't use a secure key out of the box, I suggest you start over. Factory-reset them all, plug just one of them back in. Now pair all the others back in, one by one, always using the button method. That way, they'll all connect into the same network, using a randomly generated encryption key.

Just to confirm, if I have 2 fast adapters and one slower (AV500) would the 2 fast ones still run at max speed between them or would all 3 run at half speed ?

I installed the TP-PLC utility, and it only showed the pair of PA7010 adapters with speed of 509MB between them.
Then I removed the AV500 from the outlet, and selected "Add Device" in the utility menu.
I was prompted to enter the password written on the back of the device, and after plugging the device back, it appeared on the network map with a speed of 200MB.
The speed between the 2 PA7010 dropped to 490MB.

That's how it's supposed to work With devices added, link quality goes down a slight bit, but fast devices still communicate fast.

Now proceed to add the others in.

Is it normal that 1000MB rated devices show only half speed rate between them ?

Yes. 1000 Mbit is what you get in perfect conditions; actual bandwidth decreases with the length, quality, and topology of your house wiring, and also with every single electrical device on the mains (because of the noise they backfire onto the mains wiring). 500 is actually quite OK.

Why is the RED light on when the app shows a connection speed of over 500 Mbit ?