My experience at miningrigrentals.com was a bit different than your experience at nicehash.
The rented rigs were taking some time for the difficulty to climb.
There were some alarming stats on the rig status page for the first few minutes since the hash rate was low due to the low initial diff.
But it eventually got up to speed and the hash rate reached the max for the rig once the diff automatically adjusted.
yeah mrr may not have that diff set to 1,000,000 as a requirement.
my problem with them is rental price is very high like .00001110 vs .00000980
so I don't use them.
Note bravo mining is the other small pool I promote they adjusted and I can now mine there again.
please make a port say 3334 with 2,000,000 diff for large rentals here
read from here down on the bravo mining thread he fixed his 3334 port for high rentals
https://bitcointalksearch.org/topic/m.29285675