That panel wouldn’t be a problem. Any panel that produces 6v will be safe for the Mayfly, but some cheap 6v panels actually produce higher than 6v in direct sun, so you should measure it (under load) in full sun to make sure it won’t output higher than the max input voltage of the MCP73831 (7 volts). The solar charger on the Mayfly boards v0.5b and earlier will limit the charging current to 500ma output to the battery no matter what the solar panel power output is, so you won’t damage the battery.