No prob. As far as filters go, so long as it's not causing too much aggitation for your betta fish, it should be okay. Sometimes you'll want dead spots in your tank and your plants don't really need constant circulation (don't ask me why, I just remember reading it in a few plant care forums >.>;;...). Filters help some, but they don't completely get the ammonia that's more of a job for your plants- but they don't feed entirely on amonia so you still want to keep an eye on your levels. I prefer sponge filters, but it's really not a must have.... and if you do get one or make your own sponge filter just make sure it has the right filtration strength for your size tank.
Well, I think if it were me, I'd go with sand. There would be less space for food to fit through and sit to rot in comparison to gravel. You still need to stir it every couple of months, but if you keep your pleco then it'll help AND your pleco will be much happier to be able to dig around in sand rather than gravel. Also when you siphon out debree, it's a bit easier to see the bits and pieces and, to me, a bit easier to clean up than gravel... Plus your rooted plants will have an easier time thriving. If you want to get more heavy root feeding plants, then plant specific substrate works fantastic. Although I'm not sure how your pleco would feel about it, but I'm sure it's less abbrasive than gravel/rocks >.>...
I did a little bit more digging and here's some ideas to help discourage your pleco from uprooting your plants:
"1. bundle your plants in larger groupings, and put excess substrate or a few large stones around the base of each "bundle" this will discourage your pleco (unless he's already pretty big) from digging up the roots.
2 Also observe if he keeps disturbing the same plants. They may be in his territory and he plain just doesn't want them there. I had a pleco that would shred leaves of any plant that was close to his driftwood. I moved the plant, and he hasn't looked at it since then.
3 buy plants that don't taste very good like those "onion plants" the bulb sits just above the substrate and the leaves are long and broad. apparently it tastes bitter and they'll leave it alone. I believe it's a type of lily... but it looks like a spring onion.
4 if you REALLY want a planted tank and this is an option for you... get a plastic grill (like an old under gravel filter cover) to cover the bases and roots of each plant with the leaves poking out the top, and then cover entire thing with substrate so you can't see it. It will help stabilize your plants and protect the roots from being pulled up. This won't prevent him from munching on the leaves though.
5 find your pleco a new home (like back to the LFS) and buy a different type of pleco like a rubberlip or a golden nugget. these remain small, and won'tharasss your plants nearly as much. These plecos are a little more difficult to keep than your commons, so do your research ahead of time!"
Also, I read that it helps to have more sturdy plants as pleco's like to munch on the more delicate kind. If you don't want to keep your pleco or any of your other fish, I think giving it/them to your friend with the 55g tank would be a good idea or giving it/them back to your LFS so it/they have an opportunity of finding a home that can work for it/them.