Some of you might know I've been considering keeping a betta.
Where would I buy females?
I'm thinking of a few of those for my 10 gallon, after I move my commet into a twenty gallon.

Will a UGF work for the girls?

Could I put a rubber nose pleco in with 3 -4 females?
My friend has her heart dead set on my getting a "suckerfish"

I don't know about UGF's, I don't have any experience with them. You must buy at least 5 females or else there is a great risk that they will not get along. Any less and a full hiarchy can not be estabolished, only bullying. I recomend that you get the 10 gallon all set up and attatch the new filter to the 20 gallon with your comet for 3 weeks. That way, beneficial bacteria will colonize in it and when you move the filter over to the 10 gallon with the bettas it will be pretty much fully cycled and ready. Make sure that the tank has lots of plants and hiding places because females can be pretty territorial. Most stores that sell males now sell females too. Buy them from a communal tank if you have the chance. If not, you can keep the girls in separate containers until you have all that you want. Let them go into the tank at the same time so there is less aggression over territory. They may be aggressive at first, but hopefully they'll calm down soon.

As I was planning, and as you stated, I would run the new filter with My Commet before getting any other fish.
What filters do you recomend?
I know bettas can be touchy with filters.

Hiding places I can do, I absolutely love aquascaping.
So nothing less than five females?
That means nothing less than five Hiding places.

Now I'm starting to get ideas.

I have an abundance of spectacularly shaped rocks, because there is a creek within walking distance.
I've used them in my Commet's tank, with no fuss or any PH change, after a vigorous cleaning of course.

Do I need to match colors for the females?
Its a silly question, but will they bicker if they are diffrent?

no cos they keep all sorts of colours of bettas in the tank at my lfs and there are different breeds of betta females in the same tank aswell aka plakad halfmoons and 1 lone female crowntail in the tank with them

Lol, no I don't think fish are as discriminating as humans when it comes to "color" and "race"

I told you guys it was a silly question.

Thanks though