I have had a smillerless 7g nano at my office for almost 2 years - I recently did a water change on it - last one I did was about 5 months ago - I have purple tip and green digitata (montiporas), some shrooms, and GSP's - I have the same AC 300 and only run a snall folded piece of slown-D amiracle filter pad in it and change it every month or so. SAince it's in an office, no chiller needed, I only have a heater (25w).
Toatl costs was $200 and it looks awesome
Skimmerless/Skimmer is a big debate - it all depends on so many more variable, that saying yes or no would be heresy and do it injustice because you haven't factored the other things in - In a system as complicated as these (biologically speaking) we would need to know more information before giving advice - and as Jackson said, the alternative to not doing a skimmer is frequent and large water changes
Good luck,
House