A 1.5 gallon beaker is fine. What is the shape of the beaker? Does it have a skinny neck or not? If it has a skinny neck, it's going to be a pain to clean. There should be a lot of surface water; is this the case with the beaker? The betta does need to get to the surface for air. I think I remember beakers being round and tall...ideally a betta bowl should be longer or wider than it is tall, but this beaker is ok as long as there is enough air surface for the betta to get to.
I just used regular aquarium gravel in my bowls. It's Estes brand, and I bought it in the 5 pound bags. I think I've seen some gravel in small little packets, just for betta bowls, so you don't have to buy a huge bag. But I don't have any real plants. I don't know what kind of substrate they like. But from other posts about plants that bettas like, I think a java fern would be ok. Maybe it will do well in normal gravel, but you might have to give it a few drops of a supplement.
I would get an ammonia test kit, if you don't already have one, and test his ammonia. You may find that you have to do 50% water changes twice a week. The real plant may help with the ammonia level; I don't know because I haven't had one yet. Just do your ammonia testing so you can see about how many days it takes to rise, and do your changes based on that.
Have you had a betta before? You'll love them. Mine are full of personality!
