So will that setup support me using p2p heavily in addition a second computer which I remotely control quite often and still give me QoS for the Vonage line?
Yes. That hardwared should be ample for any home setup.
Also does any happen to know if I can turn the RT31P2 into a switch without messing up the Vonage side of the house on it?
Your voip gateway's WAN must be plugged into your network, ie, mono's LAN, to function for voice. Any devices you plug into the RT31P2's LAN will be subject to its limitations whenever they access the internet.
If you're lacking a switch and you need multiple devices on your LAN but only one or two of them are using p2p, then you could add an second or third LAN interface to monowall (called OPT1 and OPT2 by default) and connect your p2p machines directly to these with crossover cables, connecting the remainder of your equipment to the RT31P2's LAN.
db