Are you an amateur photographer or trying to run a business? The answer may differ between the two.
1. Is it bad if I shoot only female models or does it matter?
If you are an amateur, then shoot whatever interests you.
If you are running a photography business, then it's not bad if it's inline with your business plan. If you don't have a business plan, then you may wish to develop one.
2. A model wanted to have a TFP portrait session with me but I realized that he didnt show up and ruined my studio session few years ago. Since then, we didnt contact each other for few years. Do I need to care about what he did or ignore his past?
The issue here is that your only experience with the model suggests he is unreliable.
Personally, I would not consider an unreliable model if I had an alternative.
3. Few friends asked me to take pics of them for their uses but not beneficial for my uses. Should I ask them for payments?
If you are an amateur, then do whatever you like.
If you are running a business, then you need to do a cost/benefit analysis.
On one hand you will gain some additional experience. I don't know you background so I can't tell you whether or not you need additional experience.
The downside of working for free is that you build a reputation as the "free guy". That makes it more difficult to attract paying clients.
Furthermore, whenever you do free work for someone, you are almost guaranteeing that you will never get them as a paying client. In their mind, the value of your work is nothing (no matter how good your images). When you reach the point that you demand payment from them, they will dump you and hire someone else. After all, if they have to spend money, why waste it on the "free guy"? They will spend it on someone who is good enough that they don't have to work for free.
4. If we didnt write a release form after we had a portrait session, what would be the biggest problem?
In the USA you generally need a model's permission in order to use their likeness to promote goods and/or services. If you are running a business and want to use the image to promote your business (perhaps on your web site), then you really should have a release.
If you are not running a business, then this isn't an issue. However privacy might be. If the model had a reasonable expectation of privacy (perhaps she was not fully clothed), then you should have written permission to demonstrate that the model has approved your publication of the images.
5. For professional photography or project uses, how and where can I find models from? (Except Model Mayhem, Craigslist, and Facebook) Do I need to find a model agency directly?
It depends on the project.
If your business is portraits or personal photography, then you advertise and your clients pay you.
If your business is model portfolios, then look at your business plan. Established models seldom pay for images for their portfolio. You need to advertise for people who wish to become models, and are willing to pay to develop a portfolio.
If you are an amateur, then you can hire models, or simply ask people. If you sound genuine, then people may be willing to model for you in exchange for images.
6. For the test shooting, do I need to ask a model directly or model agency? Can agency model work with me without an agency's permission?
It depends on what you mean by "test" shooting. If no one is being paid, then the model is typically allowed to "test" with you, however she probably isn't allowed to sign a release.
Obviously, it depends on the specific agreement the model has with the agency, and how strictly she wants to follow it.
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The important question is why are you asking?
Do you need a model to shoot an advertisement or catalog photo for a client? If so the easy answer is to call an agency, hire a model, and bill the client.
Are you just wanting to shot photos for your private collection? If so then explain to people on the street that you are a struggling photographer looking for models. Tell them you think they are perfect for your next project, and ask if they would like to pose for you. People generally respond well to sincere flattery.
Are you looking to practice your skill? Find another new photographer, and take turns posing for each other.
Is there a local art school? See if there are students there who are willing to model for you. You can compensate them by photographing their work for them.