I would do it now, if you plant the tank then you will disturb the
substrate - so best do it when there are no fish. I would get plants that do
well in Low Light (you probably have a reflector as well - and its the
intensity of light at the substrate that really counts).

Java fern is a good one as is most of the cryptocryne varieties....I have
also had a good relationship in a smaller tank with Amazon Swords (they
needed an awful lot of pruning) also some Vallisneria. I would stay away
from red-coloured plants - they look nice but in my opinion they require
high lighting conditions to grow well. If you can, only go for plants that
YOU KNOW are aquatic - most shops will try and sell you anything - even
plants that can exist in water but are not true aquatics and they die after
about 4-5 weeks - its too late then to blame the shop as they will blame
your tank and water conditions and everything else.

Good luck.