The charts on Amazon are from firmware 2.2x, the 3.0x firmware unified most of the functionality across the models .. that's the chart I gave you.
Yes USG20 can be sufficient for you if the other specs are right. Mind the throughput in the charts is one-way and the UTM performance generally sucks (see here »USG200 speed tests #3
). But if you don't need UTM then VPN and stability are very good. I personally have USG200 and can't complain (don't use UTM though).
You may consider USG50 if you ever need dual-WAN.
Yes, IPSec/L2TP VPN between various vendors and USG typically works without any issues providing the other party has standard implementation and not something proprietary funky.