By "ammonia busters" we mean that they eat up a lot of ammonia; before it turns into NitrITE or NitrATE. =] They shouldn't cause any swings if it's been planted for a while. If you suddenly plant heavily it'll usually cause a GOOD swing towards zero for all those harmful substances.
It shouldn't cause PH swings but if you already experience PH swings you'll want to buy some crushed coral and add it to your substrate. The reason being that PH usually only swings in soft water and plants need harder water to do well. =]
Driftwood depending on the kind will cause a swing in PH but usually it'll just make your water a bit more acidic and it only swings once. If you establish your plants and tank first with the driftwood in there it shouldn't be a problem because your fish will adjust to the PH.
Resin is nice especially if you're avoiding tanins but tanins are good for fish anyway and bettas love them. They're good for their scales and fins. =]
You can plant all at once if you want to. =]
The main things that you have to be careful about with plants is:
You have to be educated. You need to know what they need to thrive.
Some require nutrients at the roots. Some take it in through a rhyzome. Some take it in through their leaves and stems. Thus different plants need to be fed/fertilized in different ways.
Some have different light level and nutrient level requirements. So you'll want to get plants that are on the same level.
A good rule of thumb for low tech tanks is 2.5 watts per gallon. You want it on 8 or so hours a day with a 4 hour break some time in the middle. You can achieve this by using an outlet timer and plugging your light into that.
There are also many kinds of substrate you can choose from. This can possibly affect what you grow but luckily today we have root tabs so even if you have a jar with nothing but plain old gravel and a bunch of stems and you still want to have a plant that gets nutrients from its roots you can buy root tabs for it and you don't have to get fancy substrate.
Then there's the matter of choosing whether or not you want to fertilize further with CO2 or liquid carbon and other liquid fertilizers. =]
Here's a good beginners guide to plants.
Pretty much all of these plants are on a similar light level. They're mid level light to low level light. =]
The kind of light that you'll want for your tank is a 6,500 K(elvin) CFL(compact fluorescent/swirly) light bulb (two in your case).
Here's what they look like:
Though keep in mind that many different light bulb brands sell these bulbs. This is just an idea of what to look for. =] It should state somewhere on the packaging that it's 6,500 K(elvins) or higher. You just gotta look around for it. =]
For a 20 gallon you'll want two 25 watt bulbs. This way you have 50 watts total and perfectly 2.5 watts per gallon.
You could go with more light but if you go with more light you'll want more nutrients in the substrate and the water so that your plants can grow to their full potential in the higher light levels.
Upgrading like that also opens up the opportunities for more difficult plants. =]
I'd just stick to what's in the basic plant guide if this is your first time planting though. =]
After you've gotten the hang of it you can add more plants to the same tank at any time. =]
Here are some great online resources for planted tanks and aquatic plants in general. =]
The Planted Tank
Aquatic Plant Central (APC)
Both of those sites are forums AS WELL AS dedicated plant webdomains. You should check out the plant guide (the plant tab) on The Planted Tank. =] It's where I get a good amount of info about plants before I buy. =]
The best thing about those forums is that if you make some good friends, some people will be willing to send you trimmings of their plants for free! So you get free plants! =]
And here's a pretty good place to buy them online. =]
Don't buy floaters from there though. >:/ The price is a ripoff and I got some mixed in with my plants anyway (only a little bit and unintentionally) and they grew into what is now an extremely annoying carpet. >:/ If you want an actual floater I'd buy some Amazon Frogbit off of E-bay. I can't remember the seller but they sell it for $3.00. Can't beat that price unless you know someone that'll give it to you for free.
Another great resource on plant information (First Hand) and for getting insanely cheap plants and livestock of all sorts are Aquarium Societies and Clubs! =] Usually each state has at least one and they meet at least once a month. They'll have lectures, social activity, and auctions so you can learn, make friends, and get livestock from fish to invertebrates and plants for REALLY cheap! =] It's worth checking out! =]
Good luck and if you have any more questions feel free to ask around! =] There are a bunch of people with planted tanks here now varying in technical difficulty and skill level so someone's bound to be able to give you a hand. =]