It honestly depends on the betta girly's attitude and disposition.
Some are just as, if not more agressive than the guys, where as some are stressed way too easily. You probably wouldn't want one with either of those personalities. However, for the most part, girls are chill when added to already established community tanks. Now, if they were to added first and then have other fish added to 'their territory,' that might be an issue.
So long as the introductions go well, you should be fine. Make sure there are hiding spots for the girl as well as any other fish. Watch for aggression towards the guppies (I doubt there will be much if any at all). Watch for agression from the other fish towards the girl.
oh, and qt your girl for at least a week, especially if she is coming from a petstore. Sometimes they can carry illnesses that don't show up right away. If it's easier for you, just use a large tubberware (maybe 1/2 a gallon) and float it in the larger tank, of course completely cleaning out the qt tubberware at least once a day. This way she'll get used to the other fish as she is qt'd.
Hmmm, I don't know much about clown loaches, but from a bit of research, apparently they grow pretty large (about 16-17 inches apparently) and need a lot of room, especially since they are schooling fish (so you'd need a group of 5 or more...) This site says they need at least 75 gallons.... considering they get so large, I'd follow that, unless you can ge more information.
You'll probably want to find experienced owners of clown loaches on the sister site ( www.tropicalfishkeeping.com ). You can log in with the same login for here, and ask around for more information.
Yeah, most of the species of the fish you have right now need bigger shoals.
Clown Loaches need to be in groups of 5 to 6 minimum, preferably 10 or more. If you buy just a pair like you have, the dominant one will harass the weaker one, maybe even preventing the other one from feeding. I believe the number of Guppies you have is fine, but it won't hurt to up the number on them too, to maybe around 6 or so. Red Fire Dwarfs is just a different colouration of Dwarf Gouramis I think, seeing how you have a pair they should be fine if you have lots of plants and hiding places and cover. They're not really good community fish, they're shy and territorial, also along with noticing you have Gouramis in your tank
Since you have Dwarf Gouramis in your tank, you cannot house a female Betta.
I dunno what Long Tail Red Skirts are. ._______.;
So yeah, there you go. After taking a closer look into this, I would not advise you adding a Betta because you need to stock up on more of the same species to maintain a happy shoal of each type of fish you have already in the tank, and also you have Gouramis, they don't go well with Bettas.