28ltrs aka 7.4 US Gals is perfectly fine for a betta, I think Shinli is slightly confused with the size preference.
For a sorority, the only reason I would say it is fine, is that the more girls, the less fights. Females seem to prefer swimming in the plants (well, mine do).
It's all about experience, yes, you have to go with the tried and tested approach too, to see what works for you but that is what makes experience all the better.
I personally believe betta vases and bowls under 17ltrs are too small for a betta but that's through experience too. If you want a happy and healthy fish, research is the key, wait an extra month to build a few more pennies up to get that extra couple of litres. There are some good deals on ebay for 2nd hand tanks too, I got my firs 30ltr off of ebay for £12 GBP, bargain, considering it may have cost £60 in a shop.
We are here to advise and yes, if we feel that something is not going to go in the favor of a fish and it's owner, we'll say. But I would personally take the advice off members who have kept females and males, more so females if you're after sorority advice. Anyone can jump in, as Assaye points out about the brigades
but if it helps and I would strongly suggest reading through past posts on members in the betta section, there is some good information here.
If you are still confused about all this, I am sure if you PM a mod, they will direct you towards the members who they feel will hand out better advice for your own comfort.
Remember, we're here to help, not judge x