Hi
....... plant choice will always depend on how much maintenance you want to be doing also the actual tank specs... ie... lighting, filter flow etc.
I'm not a lover of trickle filters, there not the best at keeping parameters in check and tank cleanliness to a maximum. Maybe invest in a secondary internal filter and or an external?, this will allow for alot more stocking aswell as aiding the filtration without it struggling and ultimately... keeps you at peace of mind.
Cardinals really aren't the best fish for cycling, they can be very fragile at the best of times in cycled tanks so adding them to a cycling tank can be asking for trouble... although i don't agree with fish in cycles, fish like platies, mollies or danios are a far better choice but you have the cardinals now so making do and keeping them as happy as possible is the only option.
Fish choices after the cycle is a hard one, what do you like?..... community, cichlid, catfish... the choice can be endless sometimes so knowing what you like would help immensely on what suggestions can be given.
plant choices for a normal lowtech tank will always be the simple ones.... things like anubias, ferns, hygro species and most mosses..... done right these can be very nice features.
Hope that helps a little, it can be hard to suggest anything without knowing any sort of preference but above gives you a few ideas to throw around
