Firstly how large is your tank? You will also want to make choices about your fish based on aggression levels too. Some gouramis’ can be quite aggressive and will chase more passive fish to death. You will want some time out area's (holes in ornaments ect) for your fish to hide from others.
If you want an interesting looking tank you'll probably want a mix of bottom, middle and top dwellers. Not all fish swim around the tank, for example Tetras will mostly stick towards the top/middle of the tank, whereas a lot of the tank cleaning fish stick towards the bottom, and will in fact only feed from the bottom of the tank so floating pellets can be annoying.
Yeah a day or two for your filter should be fine as long as you've used water conditioner first, also make sure you have the right PH for your chosen fish, goldfish ect can tolerate much more acidic water, most tropical fish will prefer a neutral PH level. Some fish like Oscars will produce a high amount of ammonia so you will need to keep a watchful eye on the PH levels in the tank, once a week should be fine.
You'll also want to ensure that you have a heater in your tank, I think a decent one only costs around $30-$40, you'd have to double check but if I remember the average tropical fish likes temps somewhere between 18-24 degrees Celsius, although some species prefer it a little warmer, I think you'd be safe in to 20 degrees area.
What type of filter are you using? Also does the tank have natural plants? Some fish will shred the plants in days (Oscars) and make the tank dirty very quickly; depending on the size of the tank you might also need some sort of a bubble pump to keep the oxygen levels high enough in the tank itself.
Well I seem to have rambled on a bit here, I have kept quite a few tanks over the years and am looking at revamping my current tank, I haven't done much with it since my beautiful Oscar died, currently it supports one ugly gold fish.
I think I'll just make a quick update to my first post rather than another;
You will lose fish, this will happen, don't get disheartened by it. You are supposed to keep each new fish, or new school, isolated in a seperate tank to your main tank for 30 days before introducing the new fish in, this makes sure they have no signs of disease. I never really did this because I found it too long and it was more of a hobby than a job for me, so if you're going to be introducing new fish fairly quickly then you will most likely at some stage get disease through the tank, you'll want to have a broad spectrum antidisease medication.
Some fish suggestions that I have found work well and don't conflict too much are:
Tetras these are great fish, especially if you have a nice overhead tank light to really illuminate them.
Silver Dollars these are disk fish, and I have always found them quite nice to watch, the are middle/top swimmers but don't mind feeding from anywehere so they're great, if they get overly large though they may become a little aggressive to the smaller fish.
Gouramis, I love these both normal and dwarf versions, although I have to mention they are not rated as overly aggressive fish, I always seem to have aggressive ones, get two or more as they tend to stick together in the tank.
Striped tiger sharks, there a few different varietys in the shark areas, they're not real sharks obviously buthave a sort top fin. They are bottom feeders and help clean the tank, non agressive and come in a few species with silver colouring to orange and black.
Eel fish, I can't remember the name off the top of my head but they basically look and swim like eels, also bottom feeders but they like to hide in sand/rocks at the bottom of the tank so often they won't be visible unless looking for them.
Oscars, my favourite fish so far, but does not work well with most other fish unless they too are agressive fish. They are agressive and can grow to quite large size if they have the space and food. They will dig up the tank rocks, make the water dirty, rip plants apart (they don't eat them, just break them) and produce alot of amonia so you have to clean the water and filter regularly. They will eat any fish small enough to fit into thier mouths and they have large mouths. You can feed them live food as well, mine used to love eating crickets and frogs.