Betta tank questions

Question
Hello!  I'm a newcomer to the world of owning an aquarium, so please bare with me as I ask a few dumb questions :)
I recently bought a male crowntail betta, and I he is being housed in a filtrated two gallon tank. After much research and a lot of hatting with the pet care associates at the pet store, I bought three Zebra Danios to go in this tank.  The fish are all doing well together, and it seems like they are all comfortable with their space.
First Question:  The pet care associate said that adding a bottom feeder, such as a Ghost Shrimp, an Oto-something algae eater (please forgive my ignorance!), a snail, or a dwarf frog.  At this time, my fish are all doing well with the space they have. If I add one of these, will it become too cramped?
Second Question: The aquarium has an undergravel filtration system which includes a bottom plate, and air stone, a riser tube, and an airline tube...and I also have a gravel vacuum.  I'm wondering, since there is no actual filter or carbon cartridge, if I need to buy something else to make sure the water is actually being properly filtered, especially since I have more than just a betta in my tank.

Thanks for taking taking the time to read my extremely long post!


Answer
Hi Tori,
Congratulations on your new aquarium. Be very careful about adding too many fish to a small tank.  You are being smart about be concerned about this.  Most pet stores will tell you to add too many fish and alot of the time they will also mix the wrong kinds together. Your current fish should get along together. You could possibly add ghost shrimp but the snail might pollute the aquarium too much for its size. The otocinclus is a very small catfish which likes algae and other vegetable matter such as zuchinni, green beans and algae wafers.  They are really cute but are sensitive to any water that is not clean.  Here is a website on keeping bettas with dwarf frogs.  They can live together but the frogs sometimes have problems getting to eat it the betta gets their food before they have a chance to eat it and they can starve.  Here is the web site-http://aquaticfrogs.tripod.com/id20.html  It would probably be better if you wanted to add too many more fish to your tank that you bought a larger tank just to be sure that it doesn't get too crowded.  Most smaller tanks seem to always use an undergravel filter.  I really don't like to use them since fish waste and any decaying bits of food can get trapped under the plate at the bottom and it can eventually cause water condition problems for your fish.  I prefer to buy a filter designed for smaller tanks that has much better filtration.  You still need to do frequent water changes and vaccum the gravel since a smaller tank can become polluted with fish waste very fast even though it has a filter.  Be sure to use a really good water conditioner such as one that also removes ammonia, nitrites and nitrates along with neutralizing chlorine and chloramine. Amquel Plus or Prime is really good.  Check the temperature of the replacement water and make sure it is the same as the tank water.  
If you buy a filter be sure you don't let the current from it be too strong since bettas don't like alot of water current and sometimes they will even hide and not eat if it is too much.  You can block some water current in your tank with floating plants.  Also, it would be better if you don't use plastic plants with a betta.  The reason is that they like to swim in and out of plants and usually their fins will get torn and shredded on plastic. A better choice is the soft fake plants or real plants.  Watch the water temperature in your tank since bettas like warm water. Try to keep it at least between 76F and 80F.  They will be more active and eat and remain healthier at these temperatures.  Best of luck!
Hope this helps!
Karen