Hi GriffSchaefHQ,
It sounds like you want to trade rather than invest - that's fine but have no illusions about what it is you are getting yourself in for. You are about to start a journey that a lot of people fail because they don't know what they are getting themselves in for. Trading is boring... if it's exciting and makes your pulse race you aren't doing in right.
Here are some pointers.
Technical Analysis is now your best friend. (Don't ignore fundamental, but the short term price movements you are talking about mean that you should be doing a lot more TA than FA). Read a great deal of material on TA to get a feel for what might suit you. What I have found mentoring people on how to trade is that there are some tools that are intuitive to people depending upon the way they interpret data.
Be aware that in Technical Analysis there are "simple" mathamatical tools for interpreting data. By simple I mean that these tools use mathamatical assumptions to simplify the data. The market however is not "simple" - it is a chaotic system in which correlation can distort these "simple" tools dramatically. It is important to understand what the limitations of the tools you intend to use are. There are also some tools that use Chaotic principles for their underlying methodologies - but chaotic math is still developing in this area and it's more of a dark art than a science at this point.
Bottom line....
1) Learn how to read price action. Do you know your HH, LL, LH, HL 's?
2) Position Sizing and Money management are not optional
3) Learn what positive expectancy is and learn how to test for it.
In answer to your questions...
1. Is there a minimum amount that usually makes starting a portfolio worthwhile?
No, you can start with very small amounts if you have a +/ve expectancy system and enough leverage.
2. Commsec or another?
Read reviews on these forums of the different providers and try out a few demo's.
3. As this is a partnership (hopefully a lucrative one) is it best to start a Co. or a trust fund or none of the above?
Is the income from your trading activities likely to push you into a higher tax bracket? Until it does a company or trust structure is likely to be prohibitavely expensive for you. I would however sit down with your partner and work out the terms to your partnership agreement. This may save you heartache later on.
4. Where is a good beginning exchange that would suit our hours (normal working hours) for day trading and should we trade on multiple exchanges?
I would suggest learning in your own backyard until you find your feet. You must walk before you can run.
Cheers
Sir O