I believe you're talking about what I would call a 29 gallon tank. . .
I'm not so good at stocking questions, but I might be able to give you a starting point. . . and that is always going to be to figure out what your water conditions are, and if they're suited to the fish you'd like to keep. It's often better, certainty easier, to choose fish that will thrive in the water that you have, rather than trying to adjust things to suit their needs. Guppies prefer harder water, while many of the other species you've listed (like Tetra) won't do as well unless the water is soft. As a general rule, it seems that the water over there tends to be very hard, so I'd recommend that you find out what the GH (General hardness) of your tap water is, and go from there.
A QT tank is
really important, because fish are put through a lot of stress during transport, and change tanks several times (usually very quickly) between the breeder and your home tank. This leaves them open to carrying many illnesses that you don't want to introduce to the other fish in your tank. (Dwarf Gourami and labyrinth fish in general seem to have a rep for carrying even more illnesses, and should be kept longer in QT to ensure the safety of the others). I'm not sure how it works in the UK, but here every shop has their own rules and warranty. Most of our local shops wiill give you a 24 hour return policy, while the larger chain stores have a longer return time of 1-2 weeks or so. But even if you are able to return the sick fish you bought yesterday, you won't be able to do anything about the sickness that he brought into your tank with him, and that could be affecting livestock that you've had for a longer time. . . Another thing to consider with regards to the QT tank is that frogs are very different creatures, and can't tolerate many medications that fish can (different fish can also tolerate different medications), so if you end up in a situation where you have to medicate a sick fishy, you might run into trouble with nowhere available for the other creatures that can't be exposed to a particular medicine. Shopping from different stores is fine, but be very aware of ALL of the tanks in the shop, and their conditions. Usually shops have all of their tanks run through the same system - so they share the same water. A fish that is suffering 3 tanks down may very well be infecting the others, even if you don't see it. I'm not trying to frighten you! In most cases fish are wonderful at taking care of themselves, and don't need to be treated at all - but it *can* be a problem, and a QT tank, at least during stocking, can be a lifesaver - especially to a beginner (like me) who doesn't always know what to look for in a sick fish.
I'm a huge fan of ADF frogs, and with my personal experience with having kept them in MY 29g community tank for a time, I think that they do best in a species-only set up. It can be very tricky to feed them with fish that feed more quickly (like guppies). Still, many people do it and do it well, just be aware that it can be a problem for them to get enough food, and that it's the right kind of food (not fish flakes)
If your water is soft enough for them, Tetra are a wonderful option in most community tanks! Since they like to shoal, it's best to keep them in as large a group as you can, though 6 is usually quoted as the minimum, I've found that they are much more comfortable in larger groups, and you'll really see them shine if you keep a good-sized shoal. Again, it depends on the type of water that you have. I think most tetra prefer soft water, but some types may be more adaptable than others to different water conditions.
To get an idea of what level of stocking your tank can handle, check out AQAdvisor.com It's a great site to give you a general idea of what you *can* do, but remember that it's a computer program - so it's not perfect!
I hope this helps a bit, I'm sure others will be along to fill in the blanks! Good luck, and kudos to you for doing things right and asking questions BEFORE you make the mistakes!