Help with OpenVPN on RT-N66U

Currently I have OpenVPN running on a Linksys WRT54GL that is on DD-WRT.

When I set this up a few years ago I went through and created all of the keys like ca.crt, client.crt, client.key, etc..... I created a total of 15 keys for like 15 users (client1, client2, client3, etc). It has been working OK but now I need to upgrade as the 54GL can't handle the amount of traffic any longer.

So I got an Asus RT-N66U and people recommended this modified firmware for OpenVPN.

Here are my questions (I just flashed to 270.26 firmware):

1. Can I reuse all of those keys I created or do I have to start over from scratch?
2. Can I have multiple users connected to the VPN at the same time? At times there maybe upwards of 10 people at a time. I saw on OpenVPN that there are only client1 and client2.
3. I currently am using tap but I saw that the OpenVPN app for iOS and Android only supports tun and not tap. How easy is it to make the switch to tun? Is it just a matter of having the OpenVPN on the Asus set to tun and the same in the OpenVPN config file?

Thanks for any advice.

Vince