The maximum speed of an ethernet port of RUTX08 is indeed 1Gbps, thus, in practice, that is what you should be able to receive as long as the connected device also has the same port speed and duplex mode.
This does not apply for WiFi, as wireless connectivity is impacted by many variables, such as frequency band, distance from access point, channel width, adjacent interfering channels, WAN bandwidth, collisions, etc.
Also, you should consider that TRB has an LTE Cat 4 modem, which has a theoretical download speed limit of 150Mbps, however, the actual performance will likely produce worse speeds.
As for your first question, you will be able to connect and use TRB140 as a failover/backup WAN source, if TRB is connected to the RUTX08 via ethernet cable to the WAN port. For failover configuration example, please watch this video.
As for the speed, there are no consistently reproducible stability issues and the device should work as intended right out of the box.