No worries.
The tank holds solids, the drainfield drains the liquids.
With a low-flow, there will be less draining, which could be a good thing.

: We are doing a total remodel of our bathroom. we plan to install a Toto Ultramax toilet. Is there any concern with low flo toilets being used in a home with a septic tank? I can't imagine that there is but I'm not a plumber and just want to make sure.