Hi there and welcome to fishforum
I think that it is wonderful that you want to care for your betta so well...so, here's the answers to your questions.
1) You don't need waste absorbing gravel. Heck, I think that this is only the second time I've heard of it
. I would just use whatever "aquarium safe" gravel pleases you. It comes in many colors (I like natural myself) and the only ones that I would avoid would be bright, neon colors that may stress your fish.
2) Each aquarium has a different "footprint" meaning the size of the base, which makes it difficult to exactly tell how much gravel you will need. I'm guessing right around 5 lbs though.
3) Smooth, glass gems are fine. It doesn't matter where they are, if you like them, go for it
4) Algae eaters are pretty much out as tankmates because although most will get along fine, I don't think there are any that grow to be less than 4" which is too big for your tank. Corydoras catfish are great tankmates, but need to be kept in groups of 6 or more. My suggestion is to get a school of 6 pygmy cories, which are smaller and would fit better in your tank. The only thing about cories is that they need a sand substrate (in place of gravel). If you go for sand the only special care it needs is to be gently stirred once per week to prevent anaerobic pockets from forming (some bacteria that live in the substrate produce harmful gasses as waste products which can be harmful if allowed to form).
5) Ok, here's where the problem may come in. If you were just going to have one fish, it would be easy as you could just put him in the tank and treat him right there if illness occurs. But, since you are going to have tankmates here's what I would do. Add your tankmates to the main tank. Get a 5 gallon bin and a heater, and pull some plastic wrap (with holes poked in it) over it as a cover. Quarantine your betta in there (you'll need to do full water changes every 3 days or so, and clean the poops off the bottom with a turkey baster daily). Then, after 2 weeks of both fish being separate, assuming there is no illness, you can put the betta in the main tank.
6) I'm not totally familiar with that brand of conditioner, but on most of them it tells you what you should do on the directions on the back. If it says it is instant, I would still give it a half hour or so to distribute itself throughout the water. You can give it a stir as well.
7) Flourescent bulbs are generally used for aquariums because of their low heat output. So, you don't have to worry about cooking your fish. I'd leave it on for about 10 hours per day, although 8-12 hours are all fine.
8) Lava rock is actually not recommended for bettas because it can be VERY sharp. Think of bettas as an accident prone person
. Their fins are pretty fragile, so anything sharp should be avoided. I would go instead for a small piece of driftwood. I know you want to get everything going, so I'd look for one with a piece of slate attached to the base so that you don't have any problems with it floating. Also, driftwood releases tannins, which stain the water a tea color, into the water which bettas love.
9) You could probably get a few more plants. Bettas love to swim through and around things, as well as rest on broad-leaved plants. A floating plant would also be nice and your betta would probably build a bubble nest around it
. If you do get driftwood, arrange it so that it forms a "cave" somewhere so that your betta will have somewhere to retreat.
10) The best way to clean the gravel is with a small gravel vaccuum. This is a cilindrical piece of plastic attached to a hose. You stick the hose in an empty bucket, stick the other end in the tank (away from your betta of course) and suck a little air from the end in the bucket (use your hand around the end so you don't put your mouth on the actual hose, yuck!) until a siphon starts and water comes into the bucket. Then, you just root around in the gravel and watch the gunk come through the hose. You want to clean well, but make sure to leave some areas undisturbed each time, because too thorough of a cleaning actually destroys the "good bacteria" which keep your tank clean.
Whew! Ok, I have one more thing to say. You're going to want to cycle this tank. Cycling is the reason that you are able to do only partial, weekly water changes, instead of frequent 100% changes. Basically, cycling is the buildup of beneficial bacteria to convert fish waste (ammonia) into nitrite, and finally into nitrate. Ammonia and nitrite are harmful to your fish, but nitrate is fine in low concentrations. Nitrates are removed during water changes. There is a sticky about cyling in the "freshwater aquariums" section that you will want to read. Don't skimp on cycling, it is quite possibly the most important aspect of a healthy aquarium.
Ok, hope that helped. If you have any more questions feel free to ask